Thanks for the response. I was going to go get a new thermostat today and try that, but last night it got so cold that I turned all the thermostats up full blast, and guess what? All three rooms heated up very quickly! (When I had previously tested, I had turned the heat on in all three rooms and for half an hour and only one of them seemed to respond.) Don't know what happened, but I'm sure glad the heat works again!