Absence of Denarius implies the third scroll has already been opened in Revelation 6?

Trying to understand the Revelation in a chronological time-line as you suggest gets nobody anywhere. Right from the giving of the visions to John until Christ suddenly returns, the book of the Revelation is depicting a gradual build-up of those events. Note - there is one scroll with seven seals on it. The risen Christ opens the seals. All of the first four opened seals on that scroll can be seen to have happened from Christ's ascension onward. The four symbolic 'horses' (Rev. 6:1-14) are still galloping around the globe, and won't stop until the rider on the white horse switches from holding a bow without any arrows, to having a double edged sword protruding from his mouth (Rev. 19:11-16).

Viewing this rider on the white horse as crowned (a King) and holding a bow but without any arrows mentioned, indicates Christ the King going forth to conquer those who are to be set free and brought into his kingdom. That started with his incarnation. No force is used on them. Those he conquers are won to him, being drawn with cords of love. But in Rev. 19:11-16 this King then comes in judgment against all his enemies and this dreadful sword has replaced the harmless bow without arrows. That is still future. Until then, he is winning willing subjects.

Compare that with the rider on the red horse, the second opened seal on that scroll. He takes peace away from the Earth (war); killing with a symbolic great sword. There certainly have always been wars on Earth, but when Jesus came he warned of future wars and rumours of wars (even a rumour of a war takes peace away.) But wars would then be a beginning of pangs of sorrows (Mt. 24:6-8) and they truly have escalated and become global. Only recently has the potential for destroying all life on Earth through a nuclear war struck terror into humanity. Wars are increasing and becoming more deadly.

That is an example of how the opened seals on that scroll build up in intensity over the centuries. You ask specifically about the third seal: does absence of the denarius indicate that the third seal has been opened? Well, there have always been famines, but what is to happen once Christ ascends? A Christian prophet, Agabus, is recorded in the Bible as foretelling a coming famine - Acts 11:28. Famines are to continue throughout the centuries until Christ returns. As Earth's billions increase, and droughts, wild-fires and suchlike also increase, famines increase. They continue to this day globally, so naming a particular coin is not the point. Famines just keep on getting worse from his ascension until he returns.

The depiction of one seal after another being broken open gives structure to the visions, but the whole book of the Revelation keeps repeating the events, seven times, building up the intensity of global events, clarifying the picture, adding principles upon earlier points in the previous sections. Repetition for emphasis. Building up to get a higher and higher view until we are no longer land-locked but getting the 'view' from heaven, looking down on the world just before Christ returns.

The matter of the third opened seal is as on-going as all the other seals. They are opened once the Lamb, in the midst of heaven's throne takes the scroll from the Father's hand and opens all seven seals. They start to be fulfilled from Christ's ascension onward, continually throughout the centuries, with increasing intensity, until Christ suddenly descends to judge the world.

The opening of the seals is not a time-line to enable us to anticipate when exactly Christ will return. They have already been opened, though "the last seven plagues" clearly indicate previous sets of seven plagues; it's just that the last seven are the most severe there have ever been. Try looking at the book of the Revelation differently - not as a time-line, but as God's way of preparing Christians in every century for the tribulations and testing they will have to experience. Knowing that God is sovereignly in control no matter that world conditions just keep getting worse, keeps our faith strong. Knowing God has predicted just what will happen, and that he is the one causing the seals to be opened and pouring out the plagues on an increasingly godless world assures us that his timing is perfect. We do not need to know when Christ will return. We just need to be prepared to maintain faith while we live, whether he comes for us, or whether we die before then. That is the purpose of the book of the Revelation: to tell the saints what is to come so that we will not fall for the great deception the evil one perpetrates. This book takes over 600 pages to go into all of that: The Revelation of Jesus Christ by John Metcalfe, http://www.johnmetcalfepublishingtrust.co.uk/contact_us.htm