Must be 3 mafia members then.
A couple other things to ponder.
Mafia will never partner with corrupted wizard since they would lose in such situation because corrupted wizard wins if it is him and mafia at the end. Like any killer role, the only way he can win is total elimination. Right now, for either of those two players to survive, they probably need to target non-players or under the radar players to avoid hitting the same target.
Getting back to Apdude's comments, I doubt the investigator would just come forth and tell anybody his role without investigating them first. Moreover, how would we know the investigator did trust you if you were the wizard and killed him? Is he going to post and tell all the players that he revealed his identity to you so that we know to vote you out if he's killed? The arguement makes no sense and is flawed. Let's say the corrupted wizard makes a lucky hit and kills the investigator tonight. Are you saying we should then vote you out tomorrow? You said that it would be a fair trade.