I've been experimenting with finding out just how big I can stretch my rectum and it looks like 2-7/8" diameter is my record. I don't have anything a solid 3" and the 'device' I used to test 2-7/8 is no longer available. But, my old penis pump works great magic on (in) my ass! It's 2-5/8 so is comfortably less than my max and I can get a full 5" deep with it. The feeling of being filled up like that is such a turn on!
Of course, the nipple where you normally attach the suction hose had to be removed and the hole plugged but it was well worth the 'loss' of the pump. It serves me much better inside rather than outside! I now have series of empty 'snowglobes' ranging in width that I use to work my ass. They range from 2-1/4" to 2-3/4" and are at least 4" tall. A nice side benefit is that they are clear and I can video them from the inside!