You start out using their typical Vf @ current rating.
Then calculate:
Rlimit >= (Vsupply - (Vf_LED_total)) / Desired_Current (this is for three LEDs wired in series, not parallel)
You are better off to use too low of a Vf in your calculation than too high.
Is your DC supply regulated, or a "wall wart"? Wall warts are not typically regulated. If you are in doubt, measure the output voltage using a multimeter or DMM before you do anything.
Actually, it would be:
Rlimit >= (12 - (3*1.85)) / 20mA
Rlimit >= (12 - 5.55) / 0.02
Rlimit >= 6.45 / 0.02
Rlimit >= 322.5
322.5 is not a standard value of resistance. A table of standard resistor values is here: http://www.logwell.com/tech/components/resistor_values.html
Bookmark that page.
330 Ohms is the closest standard value.
Re-calculating to determine current:
6.45v / 330 Ohms = 19.545...mA
calculate wattage requirement:
P = EI (Power in Watts = Voltage x Current)
6.45v * 19.546 = 126mW; we double this for reliability; 252mW. This is less than 1% over 1/4 Watts, so you can use a 1/4W resistor.
Now if you were running single LEDs with single resistors, you would calculate using just one LED's Vf as Beenthere did.
Note that the wattage requirement will be higher.