I would love to run 3 gun with a T-97 or an SL-8, and you could, you just wouldn't necessarily be competitive. There is one guy that runs an 858 at the 3 gun that I used to attend. The AR-15 is chosen for a reason. It tends to combine all of the best features that are required for 3 gun. Ergonomics that you can alter to fit you personally. Rapid mag change capability. Accuracy. Easily mounted and changed optics. Magazine availability: and the fact that you can easily get LAR mags or even Beowulf if you can find them and the competition allows them. Barrel length options from 7.5" to 20". No other platform combines all of these
As far as AR's go. I started out with a Norinco. Actually I still run a Norinco, but as time has gone on I have upgraded/modified it to fit me personally. Free-float handguard, low-profile gas block, JP enterprises silent capture buffer, Nickel-Boron BCG, Ergo stock, Magpul MIAD grip, etc, etc, etc. It's the one other thing about the AR's. You don't have to break the bank. You can buy a basic model that will do the job. Then as time/money is available you can literally rebuild it one part at a time until you have exactly what you want-a one of a kind custom rifle.