A useful publication is the IEEE Standard Test Procedure for Antennas (Std 149-1979)
You are on the right track! However, you will need a gain standard antenna, such as a horn antenna.
You don't say how far apart the 3 buildings are, or how they are arranged. Can you see all 3 within a 120 or 90 degree arc? If so, you can easily obtain what is called a "sector" antenna for the AP. These have from 10 to 16 dBi gain and cost is in the $50 to $150 range.
For the...
