They could be burned out or disconnected. Or never connected. Or on a different thermostat at one time, then that thermostat removed and disconnected. Or wired wrong.
No telling what someone may have done in the past!
Is the room warm enough with just those 3?
If you are going to try to get the others working, you would be best to double check the wattage and voltage of each (label on heater). Then check the wire size, breaker, etc. to be sure everything is as it should be to handle the load.
Also check the wattage of the thermostat. It is possible that someone kept installing an undersized thermostat and it kept burning out, so their solution to the problem was to disconnect several of the heaters.
In that case, before reconnecting the other heaters, you would need to get the correct wattage thermostat. Again people don't wire or fix things like they should sometimes.
Here you can use a calculator to determine amperage based on total wattage...
(Use single phase and 120 volts or 240 volts as appropriate.)