A Marshmallow update for the T-Mobile LG G3 started popping up about a week ago, but only in the LG PC Suite. Now, T-Mobile has made the Marshmallow OTA official. The rollout started yesterday, and should be done in the coming days. Not bad for a phone that's nearly two years old.
The T-Mobile changelog is pretty basic. It lists just Android 6.0 and WiFi calling improvements. Of course, Android 6.0 is nothing to scoff at. That includes a number of cool additions like doze mode, granular permissions, and Google Now on Tap. Okay, that last one isn't so much cool as potentially cool. Maybe someday.
The new software has version number D85130d, and you should be able to manually pull the OTA by going to the system update menu. Otherwise, it'll show up as a notification soon. Enjoy this update; odds are it'll be the last one the G3 gets.