In my last post on this subject, we took a look at the the most popular 3 gun division, namely Tac Optics. This being the USA, where choice is generally regarded as a good thing, you'll be pleased to know that there are other options available to you if you don't care for the formula of a scoped 5.56 rifle, tube-fed semi-auto shotgun and 9mm handgun.
In Limited, the shotgun and handgun choices are pretty much the same as in Tac Optics, but your rifle can either have iron sights or one, non-magnified optic. Kurt Miller, the current division champion still uses irons, so if you feel that you'd like to give 3 gun a try with a stock AR15 A2 variant, you're in good company. Swapping out the issued front sight post with a finer version is usually a good call, but as far as gun modifications go, that one is hardly going to break the bank.
Fellow iron sight shooter and friend Patrick Kelley wrote a great article on using metallic sights here and he really covers the subject in more depth than I could ever hope to. If irons aren't your thing, or you currently have a red dot sight gracing the top of your rifle, then you could join the legions of shooters who have drifted into Limited for just those reasons. The current hot setups are the Aimpoint Micro, the Leupold Prismatic and the Eotech. All have a small (4-2 moa) dot, which helps when you're engaging 8" plates at 300 yards.
Currently undergoing some transitional changes, the purest form of Heavy Metal division comprises a 12 gauge pump action shotgun, single stack 45 acp handgun and a 7.62 NATO battle rifle. This is the combo you'll need to compete at the Rocky Mountain 3 Gun event, where they call it like it is, renaming the category He Man and checking each competitor's chest for the requisite amount of hair. I usually run a homebuilt FAL, Browning BPS and Colt 1911, though M1A's AR10's and even the odd Garand can be found gracing the squads.
You could also throw a low powered variable scope or red dot on your rifle and compete in the relatively new Heavy Optics class, should your tastes wander in that direction.
The bottom line is that no matter what gear you have, there's usually a 3 gun division that it'll fit. Even if your guns span several different classes, it really doesn't matter for your first few events, just show up, be safe and have fun.