I would be willing to bet that 80+% of 3-gun shooters are using a AR-15 platform. 5.56x45mm ammo is probably the most cost effective ammunition you can get other than maybe 7.62x39mm. AR platforms only require two hand to reload unlike a M-14 and AK that need a third arm to reload quickly. All in all it is difficult to be the competitiveness of the AR-15 for 3-gun.
That said I have seen AR-10, M-14, mini M-14, AKs, and even an M1 Garands at matches. I personally would think the AR-10 would be the next best thing to and AR-15. All the same controls but with major power factor.
rambling
mcb