The Third Coffin (Orochimaru)
I don't know if this thread was posted, but I'll do it again.
Here is my theory.
I believe that Minato cannot be resurrected via Impure World Resurrection/Edo Tensei, but Kabuto could have improved the jutsu. There is a possibility of reviving people from the Death God's stomach, if Kabuto had improved it.
Orochimaru's 3rd coffin is not Minato, which is my theory. If Minato truly was sealed in the belly of the death god, then when Orochimaru had attempted to summon Minato during the time, where you have the DNA, sacrifice etc, would have ultimately failed.
Although the 3rd coffin was appeared to raise higher and higher, and Hiruzen had to throw shuriken at it to throw it back down. If the 3rd coffin were to raise, that means the summoning of this individual was successful. Orochimaru summoned the three people he resurrected and stored them in coffins. If the summoning was successful then the third coffin was not the 4th. This is further supported by Kabuto's comment of Orochimaru trying to summon the 4th and FAILED.
This further supports that the 4th cannot be summoned via Impure World Resurrection.
So what was the third coffin? We might never know.