I don't recall ever seeing a commercial buckshot load for a 20 gauge, although I haven't looked either. But 3" 20 gauge shells often replicate the performance of 12 gauge 23/4" shells with equal shot weight. That being said I suppose you could work up a 9 pellet 00 buck load for the 3" 20. I have a low opinion of buckshot on game except when used in very specific circumstances, so I think the results with the 20 gauge load would be equally as miserable as the results with the 12 gauge given the same pellet count.