Rock Band 3 is the third main game in the Rock Band series. As with previous games in the series, Rock Band 3 allows players to simulate the playing of rock music and other genres using special instrument controllers mimicking lead and bass guitar, drums, and vocals. Rock Band 3 expands upon previous games by including three-part vocal harmonies, previously used in The Beatles: Rock Band and Green Day: Rock Band, and support for a keyboard instrument; Harmonix will be providing a MIDI-compatible 25-key unit with the game. Rock Band 3 will also feature a new "Pro" mode, which are designed as a learning tool to accurately mimic playing of real instruments: guitar and bass players will have to match specific fingering on frets and strings, drummers will have to strike cymbal pads in addition to snare and toms, and keyboardists will use precise fingering across the whole keyboard. MadCatz and Fender will be manufacturing controllers and add-ons to support the Pro mode.
Source: Wikipedia, "Rock Band 3", available under the CC-BY-SA License.