How to install a 4-wire a/c & fan thermostat with only 3 wires
Replace the old thermostat wire bundle with a new bundle with 4+ wires, or
Return the new thermostat you have and obtain a wireless version instead.  (Examples:  Honeywell YTH 6320, Venstar T1100&mdash;or the Honeywell RTH 7600 needs only two wires.)


The first suggestion might be very difficult.  You may have to tear off sheetrock, or do many tricky bits of drilling and coaxing wire through.  Or, if there is a short distance between thermostat and heat plant, it could be as easy as pie if the wire is not tacked inside the wall.