That's going to be tight. A SX3 has a backbored barrel. It has a .630 bore opposed to the regular .620 20 gauge bore. I would try something like .570-.585. It doesn't have to be an expensive choke either, I used a .585 Carlson's in my SX3 and it averaged 320/220 patterns with a 1 5/8oz load of TSS 9's.