How to calculate a resistor needed for an LED
So, looking at the link, my suspicion is that the 5V output is just coming directly from the USB - so it could supply much more than what you need for a few LED's.

The basic steps to determine the resistor here is


Determine the voltage drop required, Vin - Vled = 5 - 3 = 2
Determine the current required for the LED and resistor as they are in series - 0.02A
Calculate the resistance by dividing the voltage across the resistor by the current: 2/0.02 = 100 ohms


This will also dissipate 0.04W in the resistor, which should not be an issue as standard resistors handle at least 0.125W. Also, as you are probably aware each LED should have it's own resistor.