How do I calculate the resistor value for a simple LED circuit?
You'll have to check the datasheet or measure it to know how much voltage drops over your LED. Let's say this is 2V. Then the voltage over the resistor is the difference between your power supply (3V) - voltage over the LED (2V) = 1V. To get 30 mA through the resistor (and thus also the LED) your resistor has to be 1V / 30 mA = 33 Ohm.
If your LED voltage is lower the current will be somewhat higher, but the LED shouldn't break!