I didn't have any luck finding chokes for my Saiga, but I think it would do well in 3Gun if you used high brass birdshot. The cheapest shotshells have extremely wide tolerances, so they jam on occasion. The cheapest shotshells also don't have enough oomph, about one in twenty, to cycle the Saiga fully. This causes a jam that is usually a failure to eject. High brass birdshot would work perfectly, but it costs more. And on a stage with multiple shotshell types, if that does happen, would cause a great lag time in that you'd need to have a mag with each type of shot required.
Saiga mags come in 2, 5, 8, 10, 12, and 20 round capacities. They're getting cheaper and cheaper, too, which sucks if you bought them years ago, like I did.
Also, I sold my Saiga to fund my Benelli, and it was totally worthwhile. The Benelli will run fine with the crappiest ammo and the best ammo. The Saiga will run fine with the best ammo only. I'm talking quality, not preference.
Any AR barrel will be fine. Chrome-lining is very popular, but completely unnecessary. A molybdenum-steel barrel will last longer than a chrome-lined. Having shot a 3Gun, I'd say get the longest barrel and gas system you can afford.