There are a couple ways you can do this.
LEDs are generally constant current driven devices, you need to regulate current somehow.
One way to do this is to run them in parallel with a LED Series Current Limiting Resistor. If you’re running them in parallel you need a resistor for every LED in series and then wire up all the LEDs in Parallel.
You’d want to give an LED like this something like 5V to leave some amount of voltage to be able to drop across your resistor.
For the formula to calculate the resistor value is
( Source Voltage - LED Forward Voltage) / Current
For a 5V source it would be
( 5V - 3V ) / 0.03 (for 30milliAmps)
2 / 0.03 = 66.66 Ohms
We have an LED Series Resistor Calculator for the math on this available here.
Otherwise you can run LEDs in series, ten 3V LEDs in series would be 30V then you’d need one resistor for all of them. So then your LED resistor formula would be something like
35V source voltage - 30V (10 LEDs in Series ) = 5V
5V / 0.03A = 166.66 Ohm resistor.