Fire Emblem is already a difficult series, but if you're looking for an extra challenge, you'll have to wait a bit. The Lunatic difficulty is featured in several Fire Emblem games and truly puts your strategic skills to the test. Unfortunately, this feature isn't available at launch for Fire Emblem: Three Houses and will instead be a post-release DLC for free.
From a recent Famitsu interview, Genki Yokota of Intelligent Systems states in this translated excerpt:
Separately from the Expansion Pass, we plan to release the 'Lunatic' difficulty post-release as a free update. It'll have to wait for a bit, but I hope veterans will play that too.Lunatic difficulty is an option for some of the more recent Fire Emblem games such as Awakening and Fates. This mode includes stronger enemies and a larger number of enemy units on maps, as well as increased stats and more. Right now, we aren't certain how Lunatic in Three Houses changes things up.
We also know that DLC for Three Houses will come in four different waves with its Season Pass and several free updates sprinkled throughout. As Yokota states, Lunatic is separate from the Expansion Pass, so it's completely free.
In Three Houses, players take the role of Byleth, a new professor at the Garreg Mach Monastery's Officer's Academy. You are tasked with raising the next generation, training students in different weapon styles and other manners of warfare. Command students on the battlefield with Fire Emblem's signature turn-based strategic combat and achieve victory.
Fire Emblem: Three Houses releases this Friday, July 26th, 2019 for the Nintendo Switch.