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$
http://ourworld.compuserve.com/homepages/Bill_Bowden/led.htm You can also use that. If you don't know the stats on your LED, do your best to figure them out. Usually they'll be on the packaging, unless you've sourced from a surplus or another machine, etc.
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