The OTA notification should ping your device at some point over the next day or so, after which you'll be able to proceed with the install. As ever, your handset will need to be at least half-charged, and connecting to Wi-Fi beforehand is always a smart move. Additionally, backing up important info is good practise, though you'd be extremely unlucky if anything went wrong during the update to the point of data loss.
If you don't want to sit on your hands waiting for the notification, you can always go ahead and snatch the update manually. To do this, fire up the Settings app on your LG G3 or G4, then navigate through About Phone --> Update Center --> System Updates --> Check for Update. On the HTC One M9, it's Settings --> System Updates --> HTC software update --> Check Now.
Today is the day that T-Mobile is also expected to make Marshmallow available to owners of the HTC One M9, as well, though there's no word as yet on whether it's still on schedule. T-Mo's LG G4 Marshmallow update already began a couple of weeks back, while the G3 remains on the carrier's extensive to-do list.