See my answer at:
Interpolation of Three Data Points
Rather than interpolating you should first wonder if that is a useful thing to do at all:
Would your map be better than using the average temperature for the
whole area? Unless you know that there is a physical reason why
temperature differences occur I would use the average value
everywhere, and not make an interpolation. It may well be that the
prediction is more accurate if you do that than if you use an
interpolated map. Local differences may be causing the temperature
changes rather than a large scale spatial trend.
What do you need the map for?
Where is your study area located? Are you sure you cannot get other
data for the area?