I thought about a tone too...something fairly quiet. Could I have both at once i.e. the tone goes off, they look up and see which room to go to?What comes to my mind first is a wireless door bell ringer. A bell has the advantage of calling someone without looking: I can hear things behind me, but I have to turn around to see them.
Systems exist with different chimes for front & back doors, which helps for 2 of the 3 rooms. You can put the button into your pocket; they may have to search which room you are in but help would be on the way.
We had a very obnoxious buzzer at our old hospital. One of the doctors would push down and hold it. Very effective to get people's attention, but I don't want anything loud or obnoxious as our present clinic is much smaller and I don't want clients irritated by the noise.
Maybe an LED indicator at the reception desk, a tone and LED in the lab/pharmacy and in the back would work real well.