system
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human
[Star Wars] Why was Darth Sidious capable of killing the 3 Jedis that accompanied Mace Windu so quickly and effortlessly?

The three Jedis being Kit Fisto, Saesee Tiin and Agen Kolar. 

You'd think they'd be able to put up a better fight...or at least send Windu in alone and then the other 3 could have handled Anakin.
gpt
Firstly, Sidious was an exceptionally skilled duelist.  In addition, his style was quite unorthodox, combining elements from multiple forms (such as the acrobatics of Ataru, the aggression of Juyo, the thrusts and stabs of Makashi, etc), which meant it was very difficult to defend against.  

Secondly, Sidious caught the Jedi by surprise.  They did not expect the elderly Chancellor to suddenly spin through the air at them, turning himself into a whirling dervish of death.  The Jedi's ability to see into the future with the Force had been suppressed by Sidious for several years (or even possibly decades) by this point, so they had no way to predict Sidious's sudden attack.  

Kit Fisto, Saesee Tiin, and Agen Kolar were all exceptional lightsaber duelists, only slightly below Yoda, Windu, Anakin, Cin Drallig etc.  But Sidious was just too good for them.  

As for why they didn't send Windu in alone, well, why would they?  They had no inkling that Anakin would be coming soon, and when you're up against a Dark Lord of the Sith, you better throw everything you have at him, because otherwise you're going to be in trouble.  Besides, how would the Jedi know that Windu would be able to stand toe to toe with Sidious all by himself for a lengthy period of time?