I shot a match with a friend's handgun last summer and I'm hooked on 3-gun - plan on shooting a bunch over this summer when my tax return comes and I pick up my own handgun.
Right now I'm running an AR I built, 18" heavy barrel, and a Bushnell AR .223 1-4x. The shotgun is a Remington 870 Tactical with a 7+1 capacity, 18" barrel, and a Bushnell TRS-25.
Deciding on a Glock 17 or M&P Pro 9...both have their advantages. The M&P feels better in my hand, but the Glock has better accessories and support. Really not sure. Probably will depend on what I can find on the EE for under $600.