We have 2 different practices here, both using totally separate AC servers. There are 2 different server computers. Each client computer *can* access both databases if we run another virtual OS, but we choose not to do that - it's too much of a hassle. Since 1 doctor is part time, we just have 2 client computers for his practice - one for his staff and one for the doctor. These 2 client computers only access his database and the other client computers we have access only the other practice.
I can't imaging having to do this for 3 full time doctors. Seems like a lot of extra work.