The G3 did receive multiple Android updates. See here: https://www.gsmarena.com/lg_g3-6294.php - they updated it from 4.4.2 to 6.0. If that did not arrive for your phone it might have been your carrier blocking it.
The G3 can also run Android 10 via LineageOS 17.1, it is an officially supported device, see https://wiki.lineageos.org/devices/#lg. I don't know whether it will get 18.1/Android 11 though. LineageOS 17.1 worked well with the G3, I did test that recently.
