No Fail mode is a great way to let little kids or new Rock Band players have fun and play along without breaking up the gameplay when someone fails.
To turn on No Fail mode in Rock Band 3, hit start on any menu screen. Choose Options and scroll to No Fail. Use green to check it off.
To turn on No Fail mode in Rock Band 3 during a song, hit pause. Choose Options, then use the green button to check the No Fail button. Use the red button to back out to the song.