I think the Southwest family policy only pre-boards families with children 4 and under. Your sure bet is to pay the extra $12.50 (I think) each for the first spots. Otherwise, if you check in right at the 24 hour mark, you should get a good enough spot to be together. Anything A to half way+ in the B's can keep 3 together if you're willing to move toward the back of the plane.
I am flying SW myself and found flights in general to be much more expensive than prior years, same on other airlines as well though. If you book now and the price goes down, SW MAY give you a credit for the difference if you call, but it does have restrictions, like has to be used for same person, I believe, and also within a certain time frame.