I don't know how the 1,000 mile online bonus works with regards to multiple passengers on a single online booking record, but if you are concerned about the three of you not getting on the same flight, you can use the "View Seat Maps" function of the AA.com web site to determine if there are sufficient seats for all three of you on the specific flight you want. If there are a lot of seats, I wouldn't worry that in the amount of time you take to book each seat individually (in order to each receive the 1,000 mile online booking bonus) that the flight would sell out. If there were only three (or whatever your risk threshhold for number of seats is) seats available, I may not risk it.

Good luck