You should probably link them using directional antennas with high output. Based on the setup in the pic you could use Building #2 as the base site and have the others point to it based on line of sight. The antennas should work even if there is some obstacle in the way, but line of sight to the location would be better. Security and such would be your next issue. Setting up a DMZ just in case would be something to think about.
A friend of mine had told me where to get some antennas and other equipment cheap previously. Will have to check back with him then post the info.
Knowing the solution doesn't mean knowing the method. Yet answering correctly and regurgitation are considered "learning" and "knowledge".