Birdshot smaller than BB is NOT adequate for defense against human targets, unless it's absolutely the only thing you have on hand. I can't recall how many times I've posted it here, but a firsthand, short-range encounter with a rabid coyote and birdshot convinced me for all time that birdshot is not effective for defense.
Buckshot is what you need; in 12 gauge, 00 through #1 will be just dandy. The "reduced recoil" law enforcement loads are a good choice. And you don't need 3-inch magnums, as they just are tougher on the shoulder and only throw a few more pellets downrange. For 20 gauge, buckshot loads are available from #1 down to #4, any of which should be OK.