The third horcrux does not need to be found as Harry believes, for it has already been destroyed. Dumbledore drank it, knowing that he would then become the horcrux and that Snape would have to kill him. This is why D. pleads with Snape at the end, when he knows (as he always knew) that Draco is not going to follow through on his assigned task. And it's also the reason Snape and Dumbledore quarrelled halfway through the book -- Snape didn't want to do it, and Dumbledore said too bad, you agreed to this a long time ago, there's no other way, so just do it. (Of course, having taken the Unbreakable Vow to carry out the task Draco could not, Snape would already have known that he couldn't back out. But I love him for being irrational enough to argue about it anyway.) I'm sure Dumbledore had known about that particular horcrux for years -- he just had to find the right time, or rather wait for Harry to grow up enough for it to be the right time, to go and destroy it. And all along, he's been keeping Snape in reserve for that eventuality (which is very likely the real reason he couldn't let him take the DADA position until the time was ripe -- because he knew that if he did, Snape would be gone from Hogwarts by the end of the year, thanks to Voldemort's curse on the job).
So it is not, as some people have feared, a case where Snape will have to do something massively heroic in Book Seven to redeem himself from the terrible crime of murdering Dumbledore and all the other evil things he has done as a false double agent. Because he never was a false double agent, and he did only what Dumbledore asked him to do. (The murder of Emmeline Vance is more problematic, but it is possible that Snape was merely claiming responsibility for the deed after the fact to convince Bellatrix, and that the information he gave to Voldemort did not in truth directly bring about Vance's death. Or it's possible that the information he gave did lead to Vance's death, but inadvertently.)
Okay, on to some other rambling thoughts -- some speculative, some just commentary:
I'll add more comments as I think of them.