Southwest has a first come first serve seating. Whenever I've checked in online, I've gotten A - the first boarding group which gets the best choice of seats. Even when I get the B group, I can still get a window seat if I show up at the airport to line up at the gate early enough. If you wait until you're the C group, chances are that the three of you won't be able to get seats together.
Does that apply even with luggage, though?