I know this varies from airline to airline, and from flight crew to flight crew, but in general - if I have 1 overhead-size carryon, one backpack, and one purse - will I be allowed to board the plane or will they consider the backpack an additional "carryon" and refuse to let me bring it with me?

The backpack contains my cameras (I'm a professional photographer) and I will NOT check it. I can put it under the seat in front of me, and the carryon fits neatly easily in the overhead (narrow side towards the aisle, long side from aisle to outside of the plane) so I'm not a bin-hog.

I'm looking to volunteer to be bumped, so I don't want checked bags to interfere with my options for bumping or staying overnight. I usually fly UA, sometimes SW, sometimes "whoever has the best fare".

jc