They make several different kinds of buckshot in 3.5". I've hunted with it, but I don't really like it as it kicked way harder than 3" shells, and the gun I shot them in patterned the 3" shells better. Plus the 3" shells were cheaper, so I saw no reason to use 3.5" shells in that gun. Some guns may patter 3.5" shells better though.
Federal and Winchester make 3-1/2" buckshot. We use it in 4 buck and 00 buck for 3rd shot at coyotes. Makes nice 50-60 yrd back up. Shooting it from an X2.