most ammo manufactures make at least 1 type of 3" slug, rifled and/or sabot, for the most common gauges
I.E. Winchester makes a 3" in:
sabot:
12g
rifled:
12g, 20g, 410
Federal makes a 3" slug in only 12g
Brenneke makes a 3" in:
sabot:
12g
rifled:
12g (3 types), 20g, 410