|There's a ton of wireless equipment out there. Best to do your own research and comparison's and decide on what to go with.|
If you have line-of-sight between buildings then you'll want equipment that's powerful enough to reach between buildings. Also, you'll want something that allows for encryption.
Where I work we've looked at EnGenius wireless equipment This might be what you're looking for.
As to how you do it, well, there are options. Do you want to keep all 3 LAN's separate or would you like them to communicate with each other?