Round-trip between Los Angeles LAX and Washington DC, Dulles on both JetBlue and Virgin America, flying alone, I have managed to bring on-board 1 standard carry-on suitcase, 1 CamelBak back-pack (holding my laptop in it's case and a few other goodies), 1 shoulder camera bag, and 1 tripod in a clear plastic bag which wouldn't fit in my carry-on suitcase.... and I checked a duffel bag as well.
Although officially people are only allowed to bring 2 bags on-board, I haven't encountered any situation where a flight attendant or TSA officer has asked me to check the carry-on suitcase in order to meet the limit, and I think this is in part because people will go shop at Hudson News, grab a book or a few magazines, put them in a bag, go to a burger joint, pick up a burger, place it in the bag, go on the aircraft - whoops 4 bags altogether but no one cares so long as you're willing to sacrifice your own legroom and fit everything under the seat in front of you. Keep in mind these are the same people who did not catch that I had 3 liters of outside-sourced water in that CamelBak as well (3 ounce liquid restrictions my ass!).
YMMV (especially with smaller, regional aircraft).