They can't; Muur gets one-shotted with minimal effort, Vader with some exertion, and Tenebrous is behind Sidious by two Banite generations bolstered by the fact that Plagueis and Sidious were both likely far outliers due to their midichlorian manipulation, Sidious's various clone shenanigans and the fact that he was uniquely anointed by the dark side of the Force, and the culmination of the order.
Since Sidious is the fastest of the bunch, he'll kill Muur quickly, and then he can take beat Vader and Tenebrous easily.
That's possible, but it hardly matters because Wankatine is OP.
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Palpatine was not "uniquely anointed by the dark side", lol. Plagueis felt something similar upon Tenebrous' death - presumably that's what each Banite Sith feels as they get stronger upon their master's death. Sidious, being the culmination of it all, had the greatest "anointment", that's all.