While the Gauntlet and Leaderboards will not be available immediately at the beginning of Season 3, we did receive an update about the newest game mode coming to Diablo 4 during the most recent Developer Update Livestream. During this livestream, the developers discussed something that hasn't changed since Diablo 4 first released — new Shrines.
Developer Update Livestream Summary Diablo 4 Season 3 — Inside the Game
One of the new features coming to Season 3's Gauntlet is that when players defeat a boss, a Shrine can spawn that will allow players to increase their power level and kill monsters more quickly.
Since the Gauntlet is a timed event, and the overall score is influenced by how many monsters a player has killed within this time frame, it will be important to utilize these limited Shrines wisely.
Shrines can be strategically summoned by killing bosses within the Gauntlet.
One new Shrine that was shown during the Developer Update Livestream was the Pillar of Proving. Once activated, this Shrine will resurrect all enemies within a certain radius.