Why is one baseboard heating hotter, and not controlled by thermostat
Being a 3 unit house you would have to verify the hydronic piping and which zones are which, going to which baseboards, controlled by which thermostats.  And then verify how the piping is zoned off, that all zone valves and check valves are functioning.  From the sound of it it could be ghost flow.

you would get a good answer asking this on The Wall at heatinghelp.com. They will know all the correct terminology and tell u what specifically to look for.