Yes, 'ohms law' the most basic formula in electronics, it's simply V=I*R, where V is in volts, I in amps, and R in ohms. For your purpose you need to rearrange it, to give R=V/I - you then just need to work out the values of V and I, and put them in the formula.
V is easy, you have 12V in , and 3V out - so you need to drop 9V across the resistor.
I is more difficult (that sounds like really bad grammer!), as you don't give any indication what the current requirements are for the 3V supply. However, the current through the zener should be substanially more than that through the load - usually about 5 to 1 is suggested. So assuming you want to draw 10mA off the 3V supply, you need 50mA through the zener diode. This gives a total of 60mA through the resistor, so I=9/0.06, which gives 150 ohms.
You also need to work out the wattage requirements for both the zener and resitor - this is another nice simple formuls W=V*I, where W is in watts. So for the resistor W=9*0.06, which is 0.54W - so you need at least a 1W resistor, and it will probably run fairly warm. For the zener diode W=3*0.06, which is 0.18W (notice I use the full 60mA, as if the load is disconnected the zener takes the full current) - so you could probably get away with a 400mW zener, although I would probably use a 1.3W one.
This should give you enough information to work out the values you require, simple zener shunt regulators like this are only really suitable for fairly low currents - for higher currents, use a transistor (or more than one) to improve the power handling.