LG has already announced their plans to update their present flagship G4 to Android Marshmallow. However the company didn't air their plans of updating their last gen LG G3.
Nevertheless a report coming from official quarters states that the company is indeed working on an update for the G3. According to a report coming from Poland LG is not only working on the Android Marshmallow update of the device but also plan to release it by middle of December i.e. 16th to 18th.
In fact an LG Poland executive claims that the update for the G3 is in later stages of testing.
This means the LG will be rolling out the Android 6.0 update for the G3 alongside the G4. Furthermore the update will be first rolled out in Poland followed by other regions worldwide.