Definitely get the 2nd BR AC unit, this one is usually not tied into any factory ducting. It has its own built in controls and only cools the BR.
Whether you need a 3rd unit to keep the garage cool depends, on how hot it is outside, how cool your kids need it to sleep comfortably and how easy it is to adjust the outlet flows so at night you can block the living area outlets, forcing more flow into the garage; with the BR door closed. During the peak heat of the day, if you want all 3 areas cooled when its mid 90's outside, you'll need 3 units.
