From Gamespot:
Our first question was about the inclusion of three-part harmonies, which might pose a challenge for novice players. Randall said that the Beatles game will have a practice mode--much like the drum trainer in Rock Band 2--that will help singers identify harmonies that they might otherwise have trouble spotting. Similarly to how the drum trainer works, the vocal tutorial will let you loop specific sections of a song and even play tones from a particular harmony line that you can sing along to over and over.
While three-part harmonies are available to the Pauls, Johns, and Georges out there, you won't be required to nail them in order to have a great time in the game. In fact, a single vocalist can choose to fly solo with the song, or for a real challenge, players can jump on to vocals and try to sing and play at the same time, much like the Fab Four did. At the end of a song, the game will measure which harmony part a particular singer stuck to the most. If you can execute the three parts, however, you'll get a bonus point modifier (known in the game as "Triple Fab") that will reward your vocal prowess.