I'd give it to Grievous for a majority. At his peak, he outclasses either of his adversaries as duelists, and boasts substantially greater speed than them in combat. Though Ventress and Savage both have the edge of telekinesis, I doubt Ventress would use it consistently enough to actually take Grievous out of the fight, and while I think this is something Savage is capable of doing under the right circumstances, I doubt it would happen for a majority.
Something to be addressed here is Grievous's hubris. The relatively low performances he gives in some of his TCW fights, were often a result of overwhelming hubris. Fortunately for the good General, this appears to only come in play up to a certain point in time. Ever since he failed to kill Ahsoka on Florrum, he appears to take his duels a lot more seriously. In every fight we've seen him in since then, meaning the Utapau Arc where he beat Kenobi, Son of Dathomir where he contended with Maul, Labyrinth of Evil where he held his own against Windu, and Revenge of the Sith where he was overwhelming Kenobi's defenses. His second duel with Ahsoka is the last time I can think of where his hubris gets the better of him, and from that point on is when he attains the best of his dueling feats. I doubt his hubris will hinder him in this fight, given that this is him at his peak.
So aside from the fact that I consider Grievous the greatest warrior of the three, I also believe he is the best suited for the occasion. His greater number of blades provides him with more defensive coverage, offensive viability, and unpredictability than Savage or Asajj, which should be a decent edge in a free for all such as this. Also for consideration is the tactical computers slaved to Grievous's brain via cybernetics, which are the same ones that allowed him to adapt and respond to Windu's Vaapad offensive.
So in a multiple opponent free for all such as this, Grievous is the greatest duelist, is the most unpredictable, and can adapt to the situation more easily than his opponents can, and for these reasons, I'd say Grievous wins the majority of the fights here.