You can also refer to this link for the stability and bug report of android M 6.0 for LG G3 [D858,D855].
But if you want to taste android Marshmallow in you LG G3 then you can Flash this ported custom rom by following below step by step guide.
#Before we start.
1.Make sure that you have custom Recovery installed in your device i.e TWRP or CWM.
2.Before flashing this rom make a full nandroid Backup for TWRP recovery.
#How to Install Android Marshmallow Rom in LG G3
1.Download Android Marshmallow rom.zip for LG G3 from below link for your device model.
i.Download Android M for LG G3
ii.Download Googles apps 6.0
2.Now Paste both Zip file in your device internal memory.
3.Then reboot your device into TWRP recovery.
5. Now tap on Install zip,then select Android Marshmallow G3 v2.zip file and flash it.
6.Done! Now reboot your device wait for few minutes as it takes a while in first boot.
Screen Shots of Android M6.0 running on LG G3
As the rom is not much stable so Negative comments are not welcomed.