Hey mate!
Give this link a go.
http://www.phonearena.com/news/How-t...-LG-G3_id57227

Might be what your looking for. Basically what you want is Immersive mode that's available on Stock roms like CM11 and AOKP. Unfortunetly we don't have an unlocked bootloader for this phone yet, so we don't have access to this yet. Please note the above link only works for the nav buttons. You could also check out removing the nav buttons (via tweakbox) and replacing with LMT, pie controls! works like a charm for me

have a good one!