For good, accurate 3 gun info here's a couple of no Bravo Sierra sites-
http://www.brianenos.com/forums/
http://www.3gunmatch.com/
http://www.3gunrules.com/gettingstarted.phpNote the wide variation in rules from org to org and range to range. Some matches are not 30 cal neutral, allowing the low recoil of the mouse guns to be an advantage. Others take the power factors into account and the mousekateers have to pop the target twice to count whereas the 30cal shooters only have to hit once. Some sets of rules (especially at the club level) make no differentiation between auto and pump shotguns. Try to keep up with a 10+rd Open Benelli M1 with your 870, ain't gonna happen unless the Benelli shooter has a siezure and collapses. At the big, state and national matches, these things are addressed for the most part, but rules VARY, check ahead of time. Last thing you want to do is show up expecting to compete in what you thought is a "Tactical" class only to find ONE thing about ONE of your guns has bumped you into the Open Class shark pond.
Some matches are unlimited on magazine capacity, but you'll need those extra rounds of 9mm to put down steel that the 40SW and 45ACP did in one shot. So, the extra isn't really an advantage. Many matches limit capacity to 10 rounds, so the 9 is again at a disadvantage. So, unless you don't have 10rd mags for the 1911 platform, caliber is more of an issue than mag capacity. Some matches make no rulings about rifle mag capacity, other than limiting them to 30 rds. Have any of those for the M1A? I had to do a bit of research to land some for my FAL. Even so, I have 6 normal 20rd mags and one 5 shot for those special "down low" shots where a normal mag just gets in the way.
Go compete with what you have, from your list of handguns, I'd choose something of 45 persuasion with 10 rd mags. Compete for a while on the club level and get a feel for what class you ultimately want to be in and then start buying stuff.
My personal list for Tactical/Heavy Metal
FN FAL 7.62 NATO (irons or optic, depends on local rules)
G21C (hi cap or 10rd mags, ported or non ported, depends on local rules)
or
SA1911 (always with 10rd mags)
Rem 1100 12ga (comp master specs)
Start shooting 3 gun, but I'm warning you now, it's like crack, do it once, hooked for life and can be about as expensive.