Well okay... I am once again humbled by your knowledge
Had other members (Plo Koon, Mundi, Yoda) been there, Mace would have brought them to fight Sidious.
Pretending that the other three Jedi stood a chance...
Kit Fisto's Shii-Cho form would prove ineffective against the single blade of Sidious. Agen Kolar's use of Ataru would be a hindrance. Tiin would be the best out of the three for this situation, but would be dealt with quickly. But Mace had no choice. Those three were the best that he could get at the time.
Fisto, Kolar, and Tiin are incredibly skilled swordsmen, but there are others in the Order that are better than them.