SCAR the other 2 aren't even close IMO.
I would not use a retrofit piston system on a AR if a rifle designed to be a piston rifle was available for just a little more. There are no op rod springs to heat up and fail, the SCARs piston system is a true free float system. lots of little things on the rifle show me it has been designed and refined well. A 5.56 is a 5.56, the SCAR doesn't feel much different than any other 5.56 when you shoot it, it just does what it does longer and more reliably.
Don't necessarily need a piston gun to shoot 3 gun but if those are the choices, thats my pick.