*Assuming he is talking about Irek Ismaren*
Except that Nyax was not able to to control Luke without the element of surprise. Of course that is irrelevant here because Dooku and Grievous will not be expecting that attack, and neither of them are half Vong.
1 Dooku and Grievous. The number of sabers and reflexes that Nyax possesses allows him to live for a while. In the end he has no chance against two duelists of that caliber.
2 Nyax. His force domination is too much for Dooku.
3 Nyax, if he is smart enough to mind control them. Still, the duo has a chance if they work out a good strategy. Nyax was stupid enough to just let himself be hit by several objects until he died. If Dooku could think of doing something similar (probably unlike to happen), then Dooku and Grievous would win.
One display of his power is that he mind controlled Luke Skywalker into almost walking off a building and taking his breath away for the entire process. He killed 25 Yuuzhan Vong warriors and 2 Voxyn at once, just before fighting Luke, Mara and Tahiri. He sent ton after ton of rubble into the sky and destroyed 2 YV Mataloks as he was dieing.
Sabers? Dooku and Grievous end up beating him, though it takes a while to get through the numerous sabers.
Force? Nyax beats Dooku here since he's able to control Luke, Mara and Tahiri so easily. And mind you that's NJO Luke, so that's a big feat.
All-Out? If Nyax can control the two, he takes it. If not, the other two win with saber abilities.
Personally, I think Nyax was a completely unneeded character. It's like the writers got fed up with Vong vs. Jedi stuff and wanted to throw in some "classic" Dark Side stuff.