The DLC waves are separate from the planned update that will include Three Houses’ Lunatic mode.
Of course, we all know the first DLC launched alongside the game on July 26 and includes Officer’s Academy outfits for male and female Byleth. It’s part of the expansion pass, though, so you’ll need to purchase it, then download the costumes before you can access them in the game.
The second wave is expected to launch sometime before October 31. This DLC will provide new Auxiliary Battle maps as well as “helpful in-game items, and more.” It’s still not clear what these helpful items are, though we did speculate what they could be, and there’s also no indication what the “more” might consist of.
However, the Nintendo Australia page for the expansion pass said the Auxiliary maps will feature multiple map types, which is good for those who quickly grew tired of the repeated forest and town motifs. These maps will also feature more enemies who drop items.
The third wave will include new quests and costumes, and it’s planned for release sometime after October 31 and before December 31. The plural “costumes” suggests it could provide getups for more than just Byleth, though whether these will be different for the pre and post-timeskip character models still remains unknown.
Alongside the quests and costumes will be another free update.
Finally is wave four, the one that will bring new characters, locations, and story content, plus yet another free update. This one should launch sometime before April 30, 2020.
The last expansion pass that provided new story content for a Nintendo game was Torna: The Golden Country, and it offered what amounted to a completely new game in itself, even taking up an E3 slot in 2018.
Since the new Three Houses content is planned before the big summer expo, it presumably won’t be quite so expansive and will still focus on the world and events of the main game’s storyline.
The Expansion Pass can be purchased for $24.99 directly from the Nintendo Switch eShop or from Nintendo’s website.