The 3rd secret that you unlock is the option to have a dancer in the background. This is modified in the options menu, and can be turned off or set to a new person, including DDR originals like Disco and Akira, plus the artists like Naoki and U1. The dancer will just dance accordingly to the song you are playing, but doesn't work with Music Videos.
Faster Unlocks
As the manual says, unlocks are made by earning "points" in-game. So far I only know you get 1 point per player per song. Also one point per player for accessing the Extra Stage (AA or better on your last song on Heavy Mode with Event Mode turned off). First, go into option mode, and set the play level to "easiest", the number of songs to 5, and (if you have no confidence in your game), the "song failure"-type option to "end of music". Now you're ready. Play the game, and set up to versus play. Make sure you have 2 controllers. These can be pads or hand controllers. Play any songs you want to. Make sure that both players score though. This SHOULD earn you 10 points per set. If you want to earn 12, play an easy song (7 feet or less) on HEAVY mode, and AA it. This will take you to the Extra Stage (Maxx Unlimited) on Heavy mode, reverse scroll, dark mode, 1.5x arrow speed, etc.. I can't name them all. As long as both players score, you'll earn 12 points. IF by some MIRACLE you AA THE EXTRA STAGE, you'll go into the ENCORE EXTRA STAGE or something like that (I never made it that far), which is "Kakumei" And should earn you 14 points in total. I'd suggest trying to earn 10 points a set, which will earn you 1-2 unlocks per set. "Kakumei" is unlocked at 240 points and "Maxx Unlimited" is unlocked at 110 or so.
Unlock Silent Hill: 3rd Christmas
Silent Hill: 3rd Christmas Mix is the 5th secret you unlock. WARNING: This is A LOT harder than the regular Silent Hill, for it is faster and MUCH more complex. I sugest you master the original in Heavy mode, with 2 pads if possible, then start training with the 3rd Christmas. It's THAT hard.