Could the Reaper Death Seal kill Madara

Who is in the third coffin that Orochimaru tried to conjure up?

There is a way to the fourth, but very unlikely. In the anime, probably also in the manga, Kabuto Obito taught Edo Tensei. He had Danzou's bodyguard show an example. He kills one of them, takes his DNA and begins the ritual. After it's done, the dead man stands there but doesn't move. That makes me think that there was only the body and not the soul or spirit of the shinobi. As we saw in the Chikara filler sheet, Kabuto tried to use some of the Akatsuki members but failed, saying that it was not time to use it yet. This could mean that there is a cooldown between the successive rituals, perhaps the time when his mind or soul takes control of the host. Given that, it is most likely that it was the fourth, but the ritual wasn't done that far in the past. Orochimaru finished the ritual and had Minato, but he couldn't use him right away. He tried to call him to fight, but because of the Shiki Fuujin, Minato's soul or mind did not take over the host. Orochimaru either didn't know about the seal or didn't know about the cooldown. Minato died protecting the village, but many did not know how. Orochimaru may not have known it was sealed and that caused the coffin to fail and he was surprised in the fight.