raspberry pi 3V LED. What resistor to use?
If you have to buy this one only then I would suggest to start with a 100 ohm resistor, it might not please the eyes but from there you can try tweaking. Anything higher than 100 ohms would work too but the LED might appear way dimmer. Depending on the forward voltage the LED(if greater than 3.3V) might never turn on also. 

Assuming 3V as forward voltage of LED, the remaining voltage will be 0.3 V.. the sum is equal to 3.3 V (RP3). 

So, The current will be 0.3 V/100 ohms And is about 3 mA. You can slightly reduce the resistor value for brighter LED. Do not exceed GPIO current limit of the RP3. 

Second option for brighter LED: 

You can also use a BJT and drive the base with the GPIO and drive the when te LED with a series resistor connected to 5V available on Pi.