We're flying to KC for Christmas, and this is also the first flight for my 2 year old where we will purchase a seat.

I am planning on a $220 SJC-DFW-MCI fare on AA, and wanted confirm some reasoning:

Should I buy the tickets at aa.com in three separate transactions, in order to get 3 1,000 booking bonuses? Is there a chance that I won't get all three seats on the same plane if I do this?