1 master thermostat that controls 3 units

[John Moon]

I'm looking into a Triplex but I found something weird about the house. The house is heated with gas, and they have radiators throughout the units. However, there is only 1 thermostat (control panel) for the whole building. It's in 1 of the units, and they control how hot they want it to be, and the other units either complies with the heat or can turn down or up their heat through the radiator knobs (not many renters know about this). 

If I was to take over this place, I would like to fix this issue so each tenant can control their own heat. Is this something that can be fixed? What type of contractor should I call?

Other info

- 1 heater for all 3 units. It's big enough and no complaints.