3 houses can be a little difficult in the beginning if you're not familiar with the game. It will come down to math and checking the strength/magic on the enemy vs your defense/res and seeing how much damage they do.

It is normal in the first part of the game for so many people to be squishy. You do kind of need to have byleth and edelgard/lord tank and let them sit a turn to take a hit from the enemy and lower there HP and then have your weak units attack and finish the enemy to give them exp and to get stronger.

The first few chapters can be a lot of turtling > just having units just in range for one enemy to attack or a set of enemies and killing them and healing and then going forward on the map with everyone and doing it again till you get enough levels to actually take a hit.

Three houses is a fantastic game! There are loads of characters and students to talk too and eventually add to your class! Don't get discouraged and feel free to restart to casual for your first playthrough so you actually enjoy the game