You will be assigned seats when you check in. You can check in online 24 hours ahead of time and get your seats then. If you don't check in until you get to the airport, they will assign you seats. The only time the 3 of us had trouble sitting together was when we weren't able to check in online ahead of time. That time we had 2 seats together and one in the row behind us.
I think they only have one or two models of planes. All the ones we've flown have 2 seats on one side of the aisle and 3 on the other. The seats aren't overly comfortable but it's tolerable. For some reason they always seem harder on the way home!
AirTran uses only Boeing 717's and 737's. The 717's seating is 2+3, the 737's are 3+3. And you're right, the seats ARE harder on the way home. I thought it was just me.
