Actually, never mind, the bodies used for the summons are not a spoiler, since the Third defeated the summons a few episodes ago, before beginning to try and seal Orochimaru himself.

The corpses used for the summons are Zaku and Kin, the two Sound ninja who were eliminated from the third round preliminary by Shino and Shikamaru respectivly. We assume that Orochimaru killed them to both punish them for their failure and use them in the summoning.

The third summoning corpse, which would have been the Fourth Hokage, is assumed to have been Dosu, the third member of the Sound Team, who passed the preliminary by beating Chouji but then was killed by Gaara.