Considering there are over a dozen “Fire Emblem” games that bounce around a timeline, it’s very likely that “Three Houses” already has a sequel. There’s no official timeline, and fans are split on how the actual one should look like. However, The Leaderboard has a YouTube video with some solid evidence about what a correct timeline looks like.
Basically, the maps in all “Fire Emblem” games exist in the same world, and some of the games take place on the same maps. This is particularly evident in “Shadow Dragon” and “Mystery of the Emblem,” both of which feature Marth. However, in those games, Marth has contact with characters on the continent that “Shadows of Valentia” is set on.
It can be a bit confusing, but the idea is that the titles all connect in one way or another, stemming from a war between dragons thousands of years before the games take place. Because there’s no official timeline, there are questions about whether or not “Three Houses” fits into one of the three main continents’ timelines.
One argument from TalesofDerve on a GameFaqs forum mentioned that the layout of the map is very similar to Tellius, the map in “Path of Radiance” and “Radiant Dawn.” Again, this isn’t official, but the games could be related on a timeline.