The main difference between 3mm and 5mm LEDs is their size, with 3mm LEDs being smaller and 5mm LEDs being larger. The size of the LED can impact the intensity and beam angle of the light output, with smaller sizes delivering a more focused light beam and larger sizes delivering a wider, more dispersed beam. 3mm LEDs tend to have a lower power consumption than 5mm LEDs, but may not provide as much light intensity or coverage.