The 3rd Coffin is Madara.
Check this out first:
It seems Hiruzen somehow stopped the 3rd Coffin from resurrecting, NOT THE SHINIGAMI SEAL ON MINATO!
Despite what Kabuto said, he doesnt make sense, since what Hiruzen said here.
Anyway read my thread too.
So that having said, says Orochimaru gave Madara Mokuton, how old is Yamato? Old enough to live during the Kyuubi Attack. During that time its suppose that Yamato as a kid got his Mokuton from an experiment. He was merely a prototype for what Orochimaru had in plan for Madara.
So now Madara is revived by Kabuto(Or Orochimaru) again and it successfully worked. But what's odd is that Orochimaru doesnt give a damn. What's he up to? (Epic cliffhanger)
As for Kabuto's chat with Obito, he was lying. If he told Obito "So yes he resurrected Madara", believe me, Obito would have killed Kabuto by seeing where he was going with this.
So he used Yondaime Hokage as a decoy.
Read this too please:
You know that to do Edo Tensei you need to perform a ritual beforehand right? You need a living vessel and the DNA of the person you wish to bring back. Now imagine this scenario:
Orochimaru uses a living victim and sacrifices him, he calls on the DNA of Minato. Oddly enough Minato doesn't come back (Well because of the Shinigami Seal). Minato is then not revived.
So, after that how it possible for the 3rd Coffin to be Minato if the ritual wouldn't work beforehand? See what I mean. Kabuto was lying.
I see some people didnt read my given link to my theory. Read it please. And you will see Orochimaru gave Madara Mokuton, not Obito or the Hashirama DNA.