On Normal difficulty, play these combinations on the 1st and 2nd stages to get new characters on the 3rd stage...
Pops & Disco Queen -> J-Pop
Rap & Reggae -> Africa
Fantasy & Latin -> Classic
J-Tekno & Dance -> Rave
To get Rave you must Clear J-Tekno and Dance each with 85% "Great". If you do not pass with 85%, but you do pass, you will get the "Bonus Track".
Once you have "Bonus Track" and "Rave", clear all of the Stage 1 and Stage 2 characters in Normal mode (not in Free mode). After you have done this "J-Pop" will show up in the 3rd Stage always. Then if you win "J-Pop" (Normal in Game Start mode), you will receive "Techno-80's" on the 3rd stage next time you play.