3" 000 will work but 00 is better IMO. Keep the shots close and aim between the eye and the ear. If the shot is too far then don't take it. Not sure the set up, but if they are putting up ground blinds for hogs they need to feed close. I'd be comfortable with 20-25 yrds. 30-35 yrds is pushing it with a 20 gauge/modified choke. Just pound in his head if he shoots a pig with that scatter gun, reload ASAP and be ready to shoot again without hesitation. It will knock a pig dead but you have to hit him right and needs to be in range. A pig in range will ROLL
with buckshot from a 20 or 12 gauge as long as you hit them like you are supposed to. Small pigs, you can hit in body if you want to. Big pigs, shoot them in the head/neck or good luck finding them unless you are pretty close. In other words, don't take body shots on big pigs with that gun or it will end up in disappointment. Shoot the head, settle bead between eye and ear, squeeze. Good luck.