Is there any evidence that Lent used to be a 3 day observance?

Is there any evidence that Lent used to be a 3 day observance?

The short answer is yes.

Part of the reason for it was that the Early Church had difficulties in determining the actual date of Easter. As such, the fast was short, but extremely vigorous: no food or drink for it duration.

Now if the Early Church had difficulties in determining the actual date of Easter, it stand to logic that the 40 days of preparation would be equally just as hard to calculate if not almost impossible for the whole Church.

The Bishop of Alexandria was commissioned by the Council to see to the drawing up of astronomical tables, whereby the precise day of Easter might be fixed for each future year. The reason of this choice was, that the astronomers of Alexandria were looked upon as the most exact in their calculations. These tables were to be sent to the Pope, and he would address letters to the several Churches, instructing them as to the uniform celebration of the great Festival of Christendom. Thus was the unity of the Church made manifest by the unity of the holy Liturgy; and the Apostolic See, which is the foundation of the first, was likewise the source of the second. But, even previous to the Council of Nicaea, the Roman Pontiff had addressed to all the Churches, every year, a Paschal Encyclical, instructing them as to the day on which the solemnity of the Resurrection was to be kept. This we learn from the synodical Letter of the Fathers of the great Council held at Arles, in 314. The Letter is addressed to Pope St. Sylvester, and contains the following passage: ‘In the first place, we beg that the observance of the Pasch of the Lord may be uniform, both as to time and day, in the whole world, and that You would, according to the custom, address Letters to all concerning this matter.’ [Concil. Galilae. t. 1].

This custom, however, was not kept up for any length of time, after the Council of Nicaea. The want of precision in astronomical calculations occasioned confusion in the method of fixing the day of Easter. It is true, this great Festival was always kept on a Sunday; nor did any Church think of celebrating it on the same day as the Jews; but, since there was no uniform understanding as to the exact time of the Vernal Equinox, it happened sane years, that the Feast of Easter was not kept., in all places, on the same day. By degrees, there crept in a deviation from the rule laid down by the Council, of taking the 21st of March as the day of the Equinox. There was needed a reform in the Calendar, and no one seemed competent to bring it about. Cycles were drawn up contradictory to one another; Rome and Alexandria had each its own system of calculation; so that, some years, Easter was not kept with that perfect uniformity which the Nicene Fathers had so strenuously laboured for: and yet, this variation was not the result of anything like party-spirit.

The West followed Rome. The Churches of Ireland and Scotland, which had been misled by faulty Cycles, were, at length, brought into uniformity. Finally, science was sufficiently advanced in the 16th century, for Pope Gregory XIII. to undertake a reform of the Calendar. The Equinox had to be restored to the 21st of March, as the Council of Nicaea had prescribed. - The History of Pascal Time

A fast of 40 hours or three days was the the original length of Lent in the Apostolic era. Christ’s time in the tomb was used as the original inspiration. Centuries later the 40 hours became 40 days.

Closer examination of the ancient sources, however, reveals a more gradual historical development. While fasting before Easter seems to have been ancient and widespread, the length of that fast varied significantly from place to place and across generations. In the latter half of the second century, for instance, Irenaeus of Lyons (in Gaul) and Tertullian (in North Africa) tell us that the preparatory fast lasted one or two days, or forty hours—commemorating what was believed to be the exact duration of Christ’s time in the tomb (3 days). By the mid-third century, Dionysius of Alexandria speaks of a fast of up to six days practiced by the devout in his see; and the Byzantine historian Socrates relates that the Christians of Rome at some point kept a fast of three weeks. 2 Only following the Council of Nicea in 325 a.d. did the length of Lent become fixed at forty days, and then only nominally. Accordingly, it was assumed that the forty-day Lent that we encoun- ter almost everywhere by the mid-fourth century must have been the result of a gradual lengthening of the pre-Easter fast by adding days and weeks to the original one or two day observance. 3 This lengthening, in turn, was thought necessary to make up for the waning zeal of the post-apostolic church and to provide a longer period of instruction for the increasing num- bers of former pagans thronging to the font for Easter baptism. Such remained the standard theory for most of the twentieth century. - The Early History of Lent

It not only symbolized the three days Christ spent in the tomb, it is also symbolized and reflective in the modern liturgical usage of the Easter Triduum.

The Paschal Triduum or the Three Days (40 Hours), is the period of three days that begins with the liturgy on the evening of Maundy Thursday, reaches its high point in the Easter Vigil, and closes with evening prayer on Easter Sunday. It recalls the Passion, Crucifixion, Death, burial, and Resurrection of Jesus, as portrayed in the canonical Gospels.

This question reminds me of the Fast of Nineveh.

In Syriac Christianity, the Fast of Nineveh is a three-day fast starting the third Monday before Clean Monday from Sunday Midnight to Wednesday noon during participants usually abstain from all dairy foods and meat products. However, some observe the fast more rigorously and abstain from food and drink altogether from Sunday midnight to Wednesday after Holy Qurbana, which is celebrated before noon. The three day fast of Nineveh commemorates the three days that Prophet Jonah spent inside the belly of the Great Fish and the subsequent fast and repentance of the Ninevites at the warning message of the prophet Jonah according to the bible. (Book of Jonah in the Bible). Marutha of Tikrit is known to have imposed the Fast of Nineveh, and served as Maphrian of the Orthodox Syrian Church of the East until his death on 2 May 649.

Prior to the Council of Nicea, there was extreme difficulty in calculating the actual spring equinox and as such a 40 day fast would have been an impossibility.

But, even previous to the Council of Nicaea, the Roman Pontiff had addressed to all the Churches, every year, a Paschal Encyclical, instructing them as to the day on which the solemnity of the Resurrection was to be kept. This we learn from the synodical Letter of the Fathers of the great Council held at Arles, in 314. The Letter is addressed to Pope St. Sylvester, and contains the following passage: ‘In the first place, we beg that the observance of the Pasch of the Lord may be uniform, both as to time and day, in the whole world, and that You would, according to the custom, address Letters to all concerning this matter.’ [Concil. Galilae. t. 1].

This custom, however, was not kept up for any length of time, after the Council of Nicaea. The want of precision in astronomical calculations occasioned confusion in the method of fixing the day of Easter. It is true, this great Festival was always kept on a Sunday; nor did any Church think of celebrating it on the same day as the Jews; but, since there was no uniform understanding as to the exact time of the Vernal Equinox, it happened sane years, that the Feast of Easter was not kept., in all places, on the same day. By degrees, there crept in a deviation from the rule laid down by the Council, of taking the 21st of March as the day of the Equinox. There was needed a reform in the Calendar, and no one seemed competent to bring it about. Cycles were drawn up contradictory to one another; Rome and Alexandria had each its own system of calculation; so that, some years, Easter was not kept with that perfect uniformity which the Nicene Fathers had so strenuously laboured for: and yet, this variation was not the result of anything like party-spirit.

The West followed Rome. The Churches of Ireland and Scotland, which had been misled by faulty Cycles, were, at length, brought into uniformity. Finally, science was sufficiently advanced in the 16th century, for Pope Gregory XIII. to undertake a reform of the Calendar. The Equinox had to be restored to the 21st of March, as the Council of Nicaea had prescribed. The Pope effected this by publishing a Bull, dated February 24, 1581, in which be ordered that ten days of the following year, namely from the 4th to the 15th of October, should be suppressed. He thus restored the work of Julius Caesar, who had, in his day, turned his attention to the rectification of the Year. Easter was the great object of the reform, or, as it is called, the New Style, achieved by Gregory XIII. - The History of Pascal Time

Difficulties in determining the New Moon and the Spring Equinox made calculation of the date of Easter quite hard.

How did the Ancients Determine the Day of the Equinox?

Modern computers can quickly, accurately and precisely calculate the timing of an equinox or a solstice using complex mathematical formulas and equations created and validated by scientists and engineers. But such advanced technology was not available to those in ancient times. So, how did ancient people determine the spring equinox? And more specifically, how did they determine the Hebrew Day of the spring equinox?

Several theories exist as to how the spring equinox was determined by those living in ancient times. Some claim that people in the ancient world used shadows cast by the sun onto a flat surface to determine the day of the equinox. Others claim they observed sunrises or sunsets. However, all ancient methods for determining the spring equinox had one thing in common – they were all based on empirical observation. And several of these methods could have successfully determined the Hebrew Day of the spring equinox.

From CatholicEducation.com's history of Lent:

Since the earliest times of the Church, there is evidence of some kind of Lenten preparation for Easter. For instance, St. Irenaeus (d. 203) wrote to Pope St. Victor I, commenting on the celebration of Easter and the differences between practices in the East and the West: "The dispute is not only about the day, but also about the actual character of the fast. Some think that they ought to fast for one day, some for two, others for still more; some make their 'day' last 40 hours on end. Such variation in the observance did not originate in our own day, but very much earlier, in the time of our forefathers" (Eusebius, History of the Church, V, 24). When Rufinus translated this passage from Greek into Latin, the punctuation made between "40" and "hours" made the meaning to appear to be "40 days, twenty-four hours a day." The importance of the passage, nevertheless, remains that since the time of "our forefathers" -- always an expression for the apostles -- a 40-day period of Lenten preparation existed. However, the actual practices and duration of Lent were still not h*m*genous throughout the Church.

So, I suppose that the answer is that there is evidence that it was 3 days some places, but this was never official policy of the Church Universal.

As a note: the length of Lent is not a doctrine, but a practice. While it is EXTREMELY doubtful that the length will ever change, it is theoretically possible for the Vatican to make some pronouncement changing it.