Calculate voltage by amprage so you know the led is 20ma and you have 3v of power so you divide 3v÷0.02ma=150 ohms but led have voltage drop so we need to calculate diffrently so we know the led voltage is 2v so you will minus 3v-2v=1v then divide 1v÷0.02ma=50 ohms so you put you resistor to 50 ohms and you will get a nice bright led :)
