What do the Apologists mean by three persons as "one in nature" if the Son and Spirit are not eternal?

Not all the Apologists taught the Trinity using the Stoic λόγος and not all of them have equal understanding of the divine substance.

The Following are facts about the belief of an Apologist who uses the Stoic λόγος in teaching the Trinity:

1) The λόγος is eternal just as God who has it is eternal. It means that λόγος is coeval with God.

2) In the beginning [ of creation]or before creation, God made his λόγος a subsisting person to be his Son.It means that the Son isn't eternal.

3) The begetting of the λόγος is not necessary but only happened because of God's will in creating all things and in saving people.

This sort of teaching is known as the "economic Trinitarianism."

The Latin Apologist Tertullian taught that the Son learned things in order to prepare for the incarnation and so that people might also be prepared for his incarnation( Praxeas 16:3).

This sort of theology makes the Son subordinate to the Father in its real sense of Father and Son relationship but also stretches into the divine life itself. What this means is that the Father does not lack any knowledge because he his λόγος in all his activities ad extra and hence, by having the Son, experience is not necessary for him but for his λόγος alone.

Notice the analogy used of Tertullian. He taught that the Sun cannot be seen except by its light and so is the Father by the Son ( cf:John 1:18).

The only eternal is the 'name' of God and 'God' himself as well as his substance.

The λόγος, being in God, is eternal but as proceeding from God he is still of same substance, having been of same origin but now, with God's name and subsistence as the Son.

Athenagoras ( AD. 133-190) is an Apologist yet he did not subscribe to the Stoic λόγος in teaching the Trinity:

If … it occurs to you to inquire what is meant by the Son, I will state briefly that he is the first product of the Father, not as having been brought into existence, for from the beginning God, who is the eternal mind, had the Logos in himself, being from eternity instinct with Logos. ( Plea for the Christians, Chap. 10)

Irenaeus (AD. 120-200)too is an Apologist yet he believed in an eternal Son:

180 AD Irenaeus "But the Son, eternally co-existing with the Father, from of old, yea, from the beginning, always reveals the Father to Angels, Archangels, Powers, Virtues..." (Against Heresies, Book II, ch. 30, section 9)

Also, Clement of Alexnadria ( AD. 150-215) teaches the same thing, that is, that the Son is eternal:

When [John] says, "from the beginning," the elder explained to this effect, that the beginning of generation is not separated from the beginning of the Creator. For when he says, "That which is from the beginning," he touches on the generation without beginning of the Son, who is co-existent with the Father. There was, then, a Word importing an unbeginning eternity, as also the Word itself, that is, the Son of God, who being by equality of substance one with the Father, is eternal and uncreated.(Fragments of Clement of Alexandria: Comments on the First Epistle of John from The Ante-Nicene Fathers, vol. II)

These received the teachings of the Apostles about theology in its purest form and in the next century ( 3rd century) , we see this continuation of teaching an eternal Trinity as opposed to the economic Trinity, by Gregory of Thaumaturgus, Novatian and Dionysius of Rome and in the next century( 4th century), Alexander of Alexandria and Athanasius ( + the Cappadocian fathers) all defended this ancient teaching of an eternal Trinity against the error of some of the Apologists that resulted into Dynamic Monarchianism and Arianism.

Conclusion:

The theological speculation of some of the Apologists were clearly an error. The adoption of the Stoic λόγος , therefore, is not a wise thing to do.