well I'm not 100% sure, but my first thought is that the Exile and Traya would win. The Exile and Nihilius would pair off, and the Exile would win that battle. Traya and Revan would pair off, and if left alone, Revan would win, but I think that Traya could hold him off long enough for the Exile to come to her aid. With the Exile and Traya together teamed up against Revan, Revan would lose.
yes Nihilus did defeat Kreia before, I had forgotten about that, but the Exile can do the same thing that Nihilus can, if he drains power from Revan, the power that Revan would loose to both Nihilus and the Exile, would weaken him a fair bit.
Also, remember the link between the Exile and Kreia, that prevents Nihilus from drawing energy from her since the Exile is already doing it. That was the main reason he beat her before (I think), so now that he can't do that anymore, Kreia's superior knowledge could lead her to a win. Also while I'm talking about the bond between Kreia and the Exile, lets not forget that it makes both of them stronger, they are able to move as if they are under the effects of battle meditation and fight like one person. Also, any force powers that help one of them, end up helping both of them.
So I have to stand by what I said before, the Exile could tie up Nihilus and defeat him while draining enough of Revan's power to keep him from killing Kreia. Then when Nihilus is dead, the Exile and Kreia team up on Revan and win. It would be a long hard fight and Kreia would probably die, but I think that they could pull it out.
Actually the Exile can't draw power from Revan. He has never learned how. All the Exile can do is draw power from his friends. Nihilus draws power from everybody
Thats why i always said Kotor II should have an LS ending.
Now Nihilus will draw power from Kreia only, you have to assume that he isn't a complete idiot for just once. We all agree that Revan would be shit out of both of them right? Maybe even together (would be hard but not impossible) So Revan beats the shit out of Kreia, who has an incredibly mastery over the force yes, but is not the greatest lightsaber duelist. Not to mention that Nihilus is drawing power from her.
Revan would have her down on the ground in mere seconds. He would be smart, he could use two lightsabers instead of one. Move so fast that Kreia can't hold on and just kill her, then its the Exile that remains.
The exile as we all know slaughters Nihilus easily because Nihilus loses everybody to draw from while the Exile still has his friends, being the power mad soul that he is, he will try to draw from the Exile fail and the exile will kill him. So the Exile kills Nihilus and is then standing up against the most powerful Sith/Jedi in that time. Even if he would pick up Nihilus his mask which would be a smart thing to do he would still lose to Revan.