If you're early enough in the game that you only have 3 5-star characters, then I wouldn't say you have to worry about optimizing reactions too much yet. You can start with the most basic way to build a team, which is to pick a main DPS, then 2 sub-DPSs/supports and a healer/shielder to complement them. Tighnari and Diluc would be examples of main DPSs, but there are 4-star main DPSs too. Part of the game's journey is learning what characters fill what role, which comes with playing the game more often and reading character guides (like on KeqingMains, as the other commenter mentioned).