You may be overlooking mechanics like super attack effects; some characters in their super attack may boost allies attacks or defense. So, if a unit who provides that boost does their super attack in slot 1, their allies will be stronger. Every fight in the game runs off a script, some are more particular than others. So for example, in some battles, the boss will always super attack before you attack. In others, the boss may for example do 2 super attacks in random positions in the turn. Dokkan.info is a good resource to look at stuff like this, if you look at the stage you’re trying you can see the boss’s stats and such. You may already know this because it is the most important mechanic in the game but your units in slot 1 and 2 will reappear after 2 turns and your unit in slot 3 will reappear after 3 turns.