do you know what current you need? the equation would be (12V - 3V)/current. So, in general (Source Voltage - LED Voltage)/Current. If you don't know the current, something like 20-25 mA is probably safe.
1K Resistors, I've used them with 12V before and it's fine. They should provide ample current for the LED unless it's extremely high draw. Not only that but a pack of 6, from Radioshack (U.S. Electronics Store) is something around 1.15$
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