Not a working ROM by any means, but there is an Android M preview based ROM available for LG G3. While even basic phone functions don’t work — calls, sms, etc. — it’s still worth your time if you just wanna see/boast Android M on your LG G3, in full AOSP fashion. Credit for the G3 Android M build goes to manups4e, and you can find how ROM here.
Obvious enough, it’s not meant for daily use at all. And while LG G3 is yet to see Android 5.1 update, we don’t see LG updating the device to Android M anytime soon — after all, Google is yet to release the Android M to the public, and this could happen this October with the launcher of new Nexus 5 2015.
Even then, the LG G3 Android M update should be arriving only after December, maybe in Jan/Feb next year.
Meanwhile, development for the Android M ROM continues, with little hope that it may become semi-working build with calls, data, sms, WiFi, Bluetooth, etc. working, as none of these are working right now.
Once Android M is released by Google in October, and uploaded to AOSP, custom ROMs like CyanogenMod, Paranoid Android, etc. should start appearing based on Android M. And LG G3 and many other device supported by the CyanogenMod should be receiving M through that. The wait’s long for now!