Top choice amongst serious competitors is rifle length DGI on an 18" or 20" barrel. I have two match rifles configured this way.
For matches where a shorter rifle is an advantage (hoser matches) I have both 16" and 14.5" barrels with midlength DGI.
Anything shorter than midlength is less preferable due to harsh recoil. I have 7.5" and 12.5" SBRs that I run for fun (12.5" with suppressor for night matches), but they are definitely niche guns with severe limitations in a typical 3-Gun match situation.
Piston systems just add weight with no performance benefit for 3-Gun.