I couldn't figure out a good way to edit this into Donald's answer...
There are two Corum trilogies. The first is called either "The Swords Trilogy" or "Corum: The Prince in the Scarlet Robe." The second is called "Corum: The Prince with the Silver Hand."
You are thinking of the first - the Swords Trilogy - where the three books are references to three of the gods of Chaos:
- The Knight of the Swords (Arioch, prominent in the Elric books)
- The Queen of the Swords (Xiombarg)
- The King of the Swords (Mabelode (although I'm sure I've seen him referred to as Mabel_r_ode in some other hero's book))
In the Swords Trilogy, Corum loses a hand and is given "The Hand of Kwll" - which comes from yet another god, Kwll, who is alive, but exiled, and still has some control over his hand. Kwll sometimes makes the hand do things that Corum doesn't want, e.g. killing that king you mentioned.
At the end of The King of the Swords, Corum gives the hand back to Kwll in return for his help.
In the second trilogy, Corum has made himself a silver hand which, as Donald says, sometimes does things of its own free will. Specifically (spoiler ahead...)
the silver hand ends up killing Corum at the end of the last book.