Series would be the most efficient way (I'm assuming you want all three to come on together with the 12VAC, and have equal brightness), since there would be less losses in the resistance. Measure the Vf of the LEDs; it is probably around 3V each. Then, using RMS voltages, R=(12-3*Vf)/I, so if the current desired is 10mA (for example), R=300 ohms.
Here's the trick: you need to put a diode in anti-parallel with the LEDs to keep their reverse voltage from being exceeded. This is expecially important for white and blue LEDs, which are more sensitive than green, red or amber. The power dissipated in the resistor on the half cycle the reverse diode will be conducting will be much greater than when the LEDs are on, since less voltage will be dropped across the single diode, the Vf of which will be only 0.7V, so you'll need to size the resistor wattage appropriately.
The alternative would be to use FOUR white LEDs, two each in anti-parallel. Now THAT would be sweeeeeet.
j.