Here is super easy guide with video how to flash your LG G3 with Android 5.0 Lollipop. Lollipop rooted ROM by skin1980 XDA thread Rooted LG G3 with custom recovery. (You can find root and recovery links below) Copy Android 5.0 ROM D855-20A-Official-skin198.zip and Super user BETA-SuperSU-v2.19.zip to SD card reboot to recovery backup everything just in case Wipe factory reset flash ROM and SuperSU after it. Enjoy Android 5.0 or restore 4.4 from backup if not. TeamWin Recovery XDA thread Root your LG-G3 easily with PurpleDrake-Lite! XDA thread
Sadly LG G3 just doesn't get much love from developers, there aren't really any decent ROMS, almost any custom ROMS at all.
You could try LG G3 section on XDA, though not many options http://forum.xda-developers.com/lg-g3/development