what type and size resistor?

Based strictly on your numbers, you'd use W=V*I, then V=I*R

3 watts = 12v * 0.25 amps (you could also measure this to verify)

12v = 0.25a *  48 ohms per bulb.

I'm guessing 4 bulbs total, for a total draw of 1 amp.. so your resistor size would have to be around 12 ohms.

12 volts = 1 amp * 12 ohms.

Now, the wattage rating of the resistor would be calculated as follows..

12 watts = 12 volts * 1 amp

So.. you'd need either (1) 12 ohm, 12 watt resistor, or (4) 48 ohm 3 watt resistors.

Pretty big resistors.

Jim