I personally think that Roth only needed 1 traitor, someone very close that Michael would not expect. He knew, the more traitors there are, the more of a chance Mike would find out, then Roth's whole plan to have Mike assassinated is ruined. So he chooses Fredo, the one most easily convinced and also the brother of Mike. Mike, I don't think, never ever suspected Fredo of treachery until Fredo gives it away at the Superman show. Just the expression on Mike's face makes that clear.