That depends on who has the highest advantage. T.Rex could win if it was aware on what was at stake. But Triceratops on the other hand does contain a lot of full power including its massive frill, three horns, and reputation.
there have been triceratops skeletons discovered with evidence that rexes had been workin them over. so T-Rex did eat triceratops. however if tri was in a heard it would be harder for rex to get at them. kind of like a lion with a heard of wildebeest.