Yep, it's 3". The fellow I got it from said it was 2 3/4" and I simply took it at that. Never thought to put my chamber gauge in it.
I actually had one of these 30 odd years ago. Couldn't miss with that gun if I closed my eyes. This one though, I couldn't hit a barn if I closed the door on my way in.

"There are three kinds of men
Those who learn by reading
The few who learn by observing
The rest have to pee on the
electric fence and find out
for themselves."