The Gunner is a jack-of-all-trades class, as they can do damage, heal allies, or land debuffs on their enemies using specialized rounds. They aren’t very quick, and have low health and defense, so while they are powerful soldiers, it’s better for them to pick off enemies from the back row.
The War Magus is very much like their name implies – a class that focuses on both healing and attacking. They can equip swords or staves to fulfill a better role in either aspect, but mainly work with single-target effects or damage.
Finally, the Highlander, introduced in Etrian Odyssey Untold, is a powerful frontline attacker that deals damage with their powerful, traditional spear techniques. They can imbue their life energy into their attacks for even more devastating effects.
Etrian Odyssey X releases in Japan on August 2, 2018 for Nintendo 3DS. In case you missed it, Atlus recently revealed that the game will come with all class illustrations from past games as free DLC for a month after they come out.