No, I DON'T know whether it is the exact same three Sentinals but considering VERY shortly after it showing the 3, it shows another 3 destroying the Vigilant.
I also don't think they would have found one (an older Zion) due to the fact that they are always hiding and running from the Sentinals so their access to certain parts are gonna be restricted. Even if they did find it, would they click on to the fact that its an old Zion? I highly doubt it. Their beliefs are that they are the first to be freed from the Matrix.
But you say at the end "No, it is not another Zion"
Well thats just kind of hypocritical to what you put at the beginning of your sentence accusing me of not knowing if it is or isn't. From the evidence and from the superior quality of the version I have, it looks almost identical to the main shaft in Zion, ESPECIALLY the formation of the walkways/bridges.
As you say though, it IS foreshadowing what is to come, because we also, at that point did NOT know that there were 5 Zion's before the one we know now. Not too long after it is shown, it introduces the Architect who then gives us the information relevant to what was shown, what has been done and what is possibly going to happen.
Also, back to numbers, go take a read of the Plotinus thread and read the link. The first few paragraphs explain some of the meaning of some of the numbers used in the Matrix and Matrix:Reloaded.