Heating system problem that kept happening at the house part far away from the heat boiler
Some information is needed. Is this system hot water or steam? What type radiation is installed; cast iron radiators, cast iron baseboard, in the wall convectors, copper or steel finned tube? You said it is a 3 family building so how is the heat controlled? Does each apartment have it's own thermostat or is it a single thermostat? Is the system zoned? If it is zoned, how is it zoned?  Separate pumps or zone valves or a combination of both? Has the system been updated or is it original. You need to answer the above questions so people can make informed solutions to the problem. Sorry for all the questions about your heating system but we are not at your residence and trouble shooting from far away is difficult.