What I mean by this is, stars do nothing but raise the max level of a unit, allowing it to level further, which only then makes it stronger. Take a look at these Uygurs: one is 3-star and the other is 4-star, but their stats are not too different at the same level.

3-star
4-star

So my point is:

• Be cost efficient. Don't leave an empty space in your team using low level, high rarity units. Catch some 2-stars, level them up and use them instead.

• If you're not in a situation to evolve/obtain units that are higher than 3-star, a level 35 free 3-star unit can be very useful, sometimes even better than a 3-star form of a 5-star unit. It's a nice early-game strategy and a team of level 30 units should carry you at least up to chapter 3. And when you no longer have need for them, fuse them into better units for lots of EXP.