Nothing bigger than #2 Buck in 3 inch, 20 bore....
Sure Algoma, maybe #2 Buckshot in a 3 inch, 18 pellet count, (Federal), and #3 Buck Remington/Winchester/Federal, 20 pellet count, in 2 3/4 inch 20 gauge.
Imperial used to, once upon a time manufacture larger buckshot in the gauges smaller than 12.
Unsure of 20 bore, but a friend had '0' buckshot for his 16 gauge.
Must have been a weird, staggered stack, of big pellets inside the shell.
I personally have never heard of recently made buckshot in larger sizes, than previously stated.
I once, had a Savage Model 24, that I used to fool around with.
There were reloaders, that apparently made #1 Buckshot fit, into a 20 gauge case, but IIRC they had to cut the petals off the plastic wad, to make it work.