The following will allow you to play as Android 21, Goku and Vegeta.
Play as Android 21
Complete the story mode, which consists of three arcs, with nine chapters to each arc to unlock Android 21.
Play as Goku (SSGSS)
Earn 500,000 Zeni (across all game modes), or complete the Hyperbolic Time Chamber Course in arcade mode with an "A" or "S" rank on Hard difficulty. You unlock the Hard difficulty after completing the normal version of that course. To get "A" ranks, quickly win and have all your characters survive the match. Also, complete the match with a Meteor attack to get bonus points and score "S" ranks.
Play as Vegeta (SSGSS)
Earn 300,000 Zeni across all game modes, or complete "Gravity Spaceship Course" in Arcade mode with an "A" or "S" rank on the Hard difficulty. You unlock the Hard difficulty after completing the normal version of that course. To get "A" ranks, quickly win and have all your characters survive the match. Also, complete the match with a Meteor attack to get bonus points and score "S" ranks.
This will unlock ten total colors for each character.
Alternate character colors
Complete a combo tutorial (1-10) for a character to unlock a new color for them. There are ten colors to unlock for each character.