Sidious is the MOST powerful Sith Lord argueably ever. I mean, if he was powerful enough to sit in the same room with those same Jedi, who is to say he couldn't cut them up with his saber.
Afterall he is at the 9th level of lightsaber combat, he has all kinds of Sith magic and lore, and HE DEFEATS YODA. He takes down 800+ years of lightside training, and that's extremely impressive.
If he can take that on and win, I'm willing to believe the darkside will help him conquer the three Jedi.
I accept Mace being able to fight on his own because Obi-Wan manages to do the same with Anakin; whom I argue to be better than Sidious at the point of that duel.
From KitFist0
The Jedi arrive at the Senate. Mace and his posse make their way towards the private office of Palpatine where he is awaiting them. As they enter the office, Palpatine activates an audio recording device and secretly hides his saber in the sleeve of his robe. The trap is set. The four Jedi confront Palpatine who acts surprised and fearful. The Jedi light their sabers. But, without warning, Palpatine strikes first; killing Saesee Tiin and Agen Kolar within a fraction of a second."Murder! Treason!" he shouts.Then he smiles. Putting his finger to his lips, and winking at the two Jedi, Palpatine destroys the recording device with a stab of his red lightsaber."Enough of this!" declares Sidious; and with a slash of his saber, the green head of Kit Fisto lands on the Chancellor's control centre. Mace begins his attack. The two fight, moving into the public office of the Chancellor. Outside in the rain, Anakin drives the speeder close to the window. Kit Fisto's head stares directly at him from upon the desk. Anakin lights his saber and cuts through the glass, entering the private office. When Anakin arrives at the window of the private office, he can see only bodies and Fisto's head. He cuts through the glass and steps inside. Passing through the short corridor, he enters the Chancellors public office, where Palpatine duels with Master Windu.