If they act out of character then sure they have what it takes to win. But knowing Vader he's gonna want to go into a melee with Kenobi as he did in ANH and then Mace and Yoda will wreck Sidious' shit.
I highly doubt it. Vader would want to kill Kenobi personally. His obsession with Obi-Wan is well past the point of rational. He arguably wouldn't let Sidious deprive him of the pleasure.
Also, again, if Sidious takes a second to focus on Kenobi, it could leave Yoda the opening to kill him.
Sidious could just send a general force wave out that incapacitates Obi Wan. Yoda will have to shield himself from the attack before engaging with Sidious.
1. When has Sidious ever incapacitated someone of Kenobi's calibre with a casual force wave? Even Savage he has to concentrate for a second.
2. You really think a combined force shield from Yoda and Mace couldn't deflect Sidious' wave? Yoda alone could do that.
Remember on Oba-Dia when Dooku sends a force wave and Anakin and Kenobi block it? Yoda and Mace could do the same (I mean Kenobi too but it counts for shit.)
He hasn't but he's mused upon taking apart the Imperial palace which should be more then enough power to overwhelm a force user like Obi Wan.
He could shield himself but the others too? I don't know about that. Also Mace isn't shown to know how to pool his powers with another to form a defensive barrier. It's debatable that he might be able to shield himself.
They clear.
Palpatine incaps Kenobi with a gesture in round 7, and procees to take a slim majority over Yoda. Vader downs Mace by debatably being above him in everything sans dueling and agility.
Sidious, at this point, is slightly above Yoda, as ROTJ Sidious is equal to Prime Yoda, and ROTS Yoda is past his prime.
Another point is that Vader has to fight Mace. The Sith do not want Mace getting superamped off Sidious as happened in ROTS. He cannot gain that amp fighting the relatively emotionless Vader, as Vader primarily really only hates himself.