Fire Emblem can be a very punishing series nn. With permadeath, once a unit runs out of health in combat they're dead for good. Later entries allow you to turn this mechanic off or tinker with it. In terms of entry points, it depends what software options you have: Three Houses is great but the least conventional Fire Emblem experience, so I wouldn't necessarily recommend it as a starting point. If you have the resources to start with Awakening on 3DS that's the most accessible game in terms of being able to turn off permadeath while introducing more of the modern mechanics that have persisted in the series.
If you're strictly looking for options on the Switch I think the only entry they've ported is the original NES and I...wouldn't recommend starting there. I still think Three Houses is worth playing, it's a ton of fun especially the first go around, but just be aware Engage will likely be quite a bit different!
Thanks for the reply! I don’t have a 3DS, they don’t have any entry on the wii u for example right? I’ve heard quite a lot positive comments about Three Houses and there is something about that story that seems quite intriguing! But yeah, what I’ve understood is that some battles can be looong. And that’s what worries me a bit ddd. My job and gaming doesn’t really go hand in hand haha.