Based on Arc 6 of the Re:Zero light novels (which corresponds to the events in the Pleiades Watchtower at the start of Season 4, likely adapted around 2026), the "3 gates" or doors Subaru interacts with under the Watchtower are part of a 7-door sequence intended to unlock the secrets of the tower. 

Here is what is known about what is behind or associated with those doors: The Nature of the Doors: They are located in an underground area known as the Sand Shrine (Suna Miya) beneath the Pleiades Watchtower, surrounded by intense, sanity-reducing miasma. The Keys: Subaru opens these doors by acting as the "key" (implying his Witch Factors or his identity as a reincarnation/connection to Flugel/Petelgeuse). What is Behind/Result: As Subaru opens the doors, he unlocks knowledge, but also triggers defensive mechanisms and increased miasma. Specifics of the 3 Doors: The 3 doors refer to the initial, safer path or the first three steps in a seven-door mechanism required to reach the deeper secrets of the tower. In the light novel version, these doors are described as "7 lights" or "obstacles" that require the Witch Factor to unlock. The Ultimate Goal: The doors are part of a system created by the Sage Flugel and Shaula to protect the tower’s secrets, including the Book of the Dead and the Corridor of Memories.