The Exiled was immune against Nihilus' drain, the Emperor does not have such feat, and she didn't fight the 3 together.
Also, Revan and the Exile were beaten due to Scourge's betrayal.
^This. Anyone who has played KOTOR 2 and fully understood it, knew that the Exile was only able to beat Nihilus, because he could not do his special move on her. Nihilus is a Wound in the Force. Meaning that whatever Force Power Vitiate has, Nihilus can suck it up like blue milk. The only way Vitiate might beat Nihilus, is if his power goes beyond the Force. Still, Nihilus vs. Vitiate might have a very intresting outcome.. And Kreia ( hate calling her Traya) might be an old lady, and surely Vitiate could beat her, I would pay a lot to see such a fight. I'd love to see her correcting him, how he's just a parasite and never accomplished anything for himself, simply relying on the Force. Imagine the Jedi Council using Force Sever to cut Vitiate's power, and he'd die a simpleton's death.