How can a 5V power supply power 3 LEDs at 3V each?
LEDs are typically wired in parallel, just like almost every other light on the planet (you are describing a series circuit). 3V LED + suitable resistor = 5V at somewhere around 20-50mA each. Put as many in parallel as your wiring and power supply can handle.

Based on this and some other questions you appear to be missing some fundamental concepts of electrical circuitry. Go find a tutorial on basic electronics, preferably before cooking some expensive lighting fixtures. Books by Forrest Mims are particularly good and available on AMazon.