Notcot.com has revealed the first new Mimobot in wave 3 and it happens to be Darth Sidious / Emperor Palpatine. Lots of detail and the Force lightning seems to be a cool touch. More on the rest of the wave as we hear it.
UPDATE #2: The third character linking up with Darth Sidious and Wicket in wave 3 will be the Emperor's Royal Guard, which you may have guessed from the pic above. (Click here for the announcement at Mimoco.com) Additionally, Wired.com reports that C-3PO will be making his mimobot debut at Comic Con! More info on Mimoco at Comic Con 2008 right here.