How to avoid/reduce heat for three 1W LED's?
Only two ways to reduce heat in LED applications. The first is undercurrent. Reduce the current they use by choosing bigger resistors or adjusting your current controlled source. This of course leads to less light output. Alternatively you can use PWM to simulate the brightness but still cutting out some on cycles.

The other method is cooling. Large Heatsinks are the primary passive method. Or you can throw on a fan as well. There is always Peltiers but cost is a concern with those.

You do not want the leds in parallel. You will need to bring down the 12v to 3.3v on each, and then have three times the current. Very inefficient. Keep it in series, you would only need a single current regulator and only one times the current.