For those of you who want to update your rooted LG G3 to Lollipop firmware and keep root/TWRP, you will want to install stock ROMs.
If you update to latest Lollipop firmware using OTA (over-the-air) or LG G3 stock firmwares, you will run into the problem of not being able to install TWRP due to new locked bootloaders on Lollipop. (Currently there is TWRP for Lollipop on D855 only!)
By installing a stock ROM and flashing SuperSU zip file, you will be able to keep root and your TWRP recovery so you can still flash custom ROMs.
To update your rooted LG G3 to Lollipop firmware, simply download the stock ROM files for your model of LG G3 here and SuperSU zip file here, then simply flash away using TWRP recovery (follow the video tutorial if you don’t know how to do this).
Just in case you want to go back to KitKat or the custom ROM you were on, make a backup ROM using TWRP recovery first, then install the files! Also, you do not need to do a factory reset if coming from stock firmware (or never installed a custom ROM).
If you are experiencing any lag after updating to Lollipop, you may want to consider doing a factory reset afterwards to start on a clean slate. You can use Titanium Backup app to backup all your apps, use SMS backup for SMS texts, then do a factory reset.