Minato was seal by the death reaper seal in the past while fighting Kurama.There is no way Orichimaru would have summon Minato at that time because he will need the Uzamaki mask and the uchiha naka shrine inorder to reverse the seal and summon him again.We saw in recent chapter that Minato was seal in the death reaper body alongside the other kages. http://narutobase.net/manga/Naruto/618/10
I believe the third coffin was actually Madara at the moment because minato die after Madara die because Obito turn bad and was following madara dreams.Orichimaru may be collected Madara body at that time or Madara .
What you think?
ADDITIONAL NOTE TO NARUTO FANS:
Kurama other chakra have not been release from the death reaper seal.We saw only 5 spirit like thing comming from the reaper cut bully (4 hokages and orichimaru arms).There was no sign of kurama half was comming out from the bully .This could either mean kurama other half is in the death god reaper still or it seal directly in minato body before he die. http://narutobase.net/manga/Naruto/618/10