[Hondaracer]

Only way to control all 3 would be if they had been initially wired together.

I purchased a 7 day programmable thermostat from Home Depot back in our appartment it was a Honeywell I beleive. Thing with the digital programmable baseboard thermostats is they are all pretty ugly and have limited models to choose from. The Honeywell one was nicer but quite big.

Think your best bet may be to use a programmable one in the room you frequent the most then keep the others at a temp where they aren't constantly on/off.

A huge energy drain is when your thermostat is located in a cold area so it constantly has to kick on or off, have a situation like this in our suite where one thermostat/heater is close to the entry door so cold air is constantly circling there, best to maintain a temp imo then to try and go from 10 deg to 21 throughout when you get home etc