Well, it is indeed official now - Android 6.0 has been released for the LG G3. And it is happening in Poland as we speak, but just so we're clear, it has started today, not yesterday.
From what we gather, the firmware has to be installed via a PC, and an OTA update is not available for the time being though it could happen eventually.
Both 16GB/32GB storage versions are eligible, and what looks like a soak test is so far limited to unlocked Polish market handsets.
The procedure was more or less established over the past couple of months with the G4 - initial Marshmallow release in Poland, followed by South Korea, and then Europe. So while it may take a while to reach widespread availability, you can rest assured that Marshmallow is coming to your LG G3, sooner or later.
Last Night I received the official update to Marshmallow through the LG update system. Note that it takes about an hour to an hour and a half to finish the update. Plug in the charger to the battery
How to upgrade
The update is available on xda I safetly updated my g3 d855 last night with no issue