Whether you step up or step down you will waste energy. Why not use the LiPos directly? Three LiPos will give a voltage between 12.4 and 10.8v depending on the state of charge.
Do you really mean you have an LED lamp that takes 5 amps at 12v? If so the battery will discharge very quickly. That seems more like a job for a large lead-acid battery.
This does not seem to have anything to do with Arduinos!
...R