Probably a bit late but if you have an LG G3 (and probably a G4 and G5 as well) and are suffering from the screen fading to black (looks like the power has been disconnected and the colour fades outs), then it's a known fault of the phone.
Both my daugher's (after 2 years) and wife's (2.5 years) had the exact same problem within a month of each other.
It's due to heat buildup causing the connections to the motherboard to crack as the phone heats and cools.
The drastic fix is to bake the motherboard for 10 mins at 180 degrees. This presumably melts and reconnects the solder.
I've done it to both phones and they are both working fine now.
It's probably a temp fix and I might need to do it again but the phones are usable again.
The other common issue is the infinite boot loop. That is fixed by having a new battery. My wife's phone did this immediately after I'd baked it and I thought I'd caused it. However, got a working battery from a mate (who also has the fading to black screen issue) and that fixed it.
Nice phones but terrible build flaw.