Instructions
Go back to any mission that you have already completed in any of the seven sagas in Dragon Battle Collection. This is the game's version of what typically is called "Story Mode" in most fighting games.
Look at the options when you select a battle you've already completed. You can start the battle immediately, get a recap of the story, find out what you need to do to clear the battle successfully and look at the peripheral missions for the battle.
Highlight the "Missions" option to see a list of three conditions you can fulfill to earn stars. This varies from battle to battle, but you will see things like "Land three Super Attacks in a row" or "Land a hit with a deflected ki blast." Every battle has the condition "Finish on Hard difficulty."
Fulfill the conditions as instructed for all of the battles to earn all three stars for each one.