240 volt heater installation
Maybe! but read the instructions for over-current protection of your 240 volt heater, you might have to swap out the breaker. Also, make sure the wire gauge size is good for the heater. Larger than needed is OK, smaller is obviously not. As always, wire size, breaker rating and device amperage requirements all need to be considered. But just having an extra conductor in a cable isn't an issue, to actually answer your question.  Use the red &amp; black as the hots, the bare copper or green wire as the ground and cap off the white. Double check your voltages to make sure nobody monkeyed anything up.
IMPORTANT UPDATE:  JACK mentioned the possibility of this being a 120 volt heater due to the black and white wires.   CRITICAL that you make sure this is actually a 240 volt heater. If not, well, lets just say it will get very hot very quickly and probably start a fire if over-temp devices don't trip.   Sorry for my answer...it was clearly limited to other possibilities.