Is there A mythical creature with 3 arms?

I am unsure of any ancient mythical animals, but you could try making your own race if you would want to.

The only one I have ever heard of is Lt. Arex, on the animated "Star Trek" from the early seventies. He is an Edosian; in some sources is a Triexian. Perhaps one name is the species, the other the planet he is from.