ROTS Sidious would kill both. Only reason why Mace beat him was because Sidious was holding back his LS skills, did not use Force powers, threw the fight, Shatterpoint, and Vapaad. As for Vader, his prime was ROTJ? Just thought he was always on a consistent level. and Galen in the 1st game did own by by himself, 2nd game yeah Vader was holding back and had a hole in his suit and Galen's force lighting was amp'd by Kamino's storms and the power generators.
A rebuttal isn't even needed. No way in hell is Sidious taking down two "Jedi" of power comparable to his own at the same time. Hell, we even saw him lose to one on his own. While the other left him howling and pain and Sidious only won that battle when Starkiller decided to kamikaze some Stormies trying to kill his friends....
Sidious vs. Galen is an obvious outcome. Galen's most powerful attack didn't even singe his robes, while at the same time the lighting Sidious was using wasn't at full power. I think Sidious would probably kill Galen on the get go, leaving Mace alone with Dooku and Sidious, and he dies from that encounter. Sidious wasn't howling in pain, unless you mean the gameplay mechanics of the fight. Sidious in the comic straight up laughed his ass off when Galen hit him with a huge force lightning attack.