DC3 will ship with over 40 songs. Harmonix revealed a quarter of these tracks today. As you can see, it's a mix of old and new hits:
50 Cent - "In Da Club"
Cali Swag District - "Teach Me How to Dougie"
Cobra Starship ft. Sabi - "You Make Me Feel..."
E.U. - "Da' Butt"
Edward Maya & Vika Jigulina - "Stereo Love"
Gloria Gaynor - "I Will Survive"
Marcia Griffiths - "Electric Boogie"
Usher ft. Will.I.Am - "OMG"
Usher - "Scream"
Van McCoy - "The Hustle"
Usher was actually on hand during Microsoft's E3 press conference when they announced the game. He then performed his song "Scream" for the crowd. Kind of felt like an elaborate advertisement for Usher's song but I guess I can't really complain about product-whoring in the middle of a corporate press conference.
Dance Central 3 will feature a new Crew Throwdown mode. Two teams of four players will be able to face off in a number of different game modes. For example, a Make Your Move mini-game will task players with creating fresh dance routines on the fly and outdoing their competition in the process.