My guess is the reveal will occur later in the year at either Paris Games Week (October/November) or the Montreal International Games Summit (December 2018), but E3 (June) is a good possibility too since it is arguably the most "major" of the annual gaming events. This appears to be a hidden confirmation of the long rumored title, Shadow of the Tomb Raider.
Despite The Game Awards taking place tonight at 8:30 PM EST, some companies have chose to announce some big news beforehand. While the name wasn't confirmed, we do know we'll be continuing Lara Croft's adventure very soon.More news: Chiefs suspend CB Peters one game
The following is the text of the announcement of Shadow of the Tomb Raider from the official Tombraider twitter account.
The current reboot series began with the release of Tomb Raider in 2013, followed by Rise of the Tomb Raider in 2015. Square Enix has just revealed that they have some big news to share with us soon. The statement from Square Enix did not confirm this as being the game's title, so it's entirely possible that it could end up being named something else. In a world where game reveals very often happen full years before the release of a game, this can put anybody on the edge of their seat.
Story info and a more specific release date on the third game are unknown at this time.