First off, check with your local regulations. It may be called buckshot, but in many places you're not allowed to use it for deer hunting.
I did a quick check, and the biggest I could find for a 3" 20-gauge was #2 buck, probably good for 30, maybe 40yds. I think you'd be better off with rifled slugs, certainly good for 50yds, maybe more.
Happy hunting!