How to step down from 5VDC to 3VDC?
If you're worried about burning out LEDs, then current is your problem, not voltage. You can limit the current with a series resistance (which LEDs always need).

If you're using standard 5mm LEDs, a 1k resistor for each LED will only allow a few mA through (I usually try to stay under 20mA for these).

LED Strips already have series resistors, so as long you don't apply a voltage higher than they're rated for, they should be fine.