The LM317 is rated for 1 watt without a heatsink and something like 3-4 watts with a heatsink. Use the formula R = 1.23 / I to calculate the resistor you need. R = 1.23 / 0.70 = 1.75 ohm resistor; find whatever is closest to that value. (The 0.70 is the 700 mA 3-watt LED.) Then use P = 1.23 x 1.23 / R to calculate the watts you'll have to dissipate. P = 1.23 x 1.23 / 1.75 = 0.86 watts. As long as you use a resistor with a rating greater than 1.5 watts (to be safe), you will be fine.

The above was used just to calculate the resistor value. Now you have to calculate the power dissipation of the rest of the system. Running two 3.6V LEDs; Vx = max forward voltage with alternator running - LED voltage drop - 1.23 or Vx = 15V - (3.6V + 3.6V) - 1.23 = 6.57 volts. Px = Vx x I or 6.57 x 0.70 = 4.6 watts. Yikes, you're right, I'm not sure how you safely dissipate nearly 5 watts of heat.