Please note the following:
1. It does not wipe the /data/ partition of your device.
2. It will reinstall any bloat you have uninstalled including those apps that you carrier put into /data/
3. It will remove any files you have added to /system/ including root or other modifications
4. Any files you have "frozen" will stay disabled, you cannot unfreeze them without rooting again - this guide will be for a "full unroot" so there is an optional step below to unfreeze apps.
1. Download LG PC Suite from LG's website - you must select model type before you do so, but as far as I can tell it downloads the same version regardless.
2. Update the app so it recognises the G3 - this should ask you to update automatically on opening the app, if not you can do it from the "help" menu.
(NB: if you have used some of the KDZ flashing methods on here you may have issues updating as your hosts file is probably altered to block the PC Suite gaining a connection).
3. Connect the phone via USB (the app allows for a wifi connection but this is very restrictive, you require a USB connection for this).
*OPTIONAL STEP* If you want to go back to a truly stock experience then unfreeze any apps you have frozen (I am unsure if having apps frozen will effect OTA installations, it should still work fine, in theory).
4. With the device connected go to the "phone" menu in PC Suite and select the option that says "Restore Upgrade Errors".
5. It will then scan your device, download the rom and begin flashing, this is all automatic without any further input from you, do not cancel it.
6. The phone will reboot when finished. You are now unrooted and unmodified and OTA's should work.
*OPTIONAL STEP* If you want to go back to 100% stock then its probably a good idea to factory reset your phone at this point - this will wipe your whole device (of course).
And there you have it, your quick(ish), painless method for being able to unroot to accept OTA's without requiring a wipe.