I think I have found out why this "three song rule" has come about, there are some performers who are camera shy and can't perform to their best when lenses are stuck in their faces...yes, most are not camera shy, and yet this 3 song rules apply because they felt that it was good enough for every performer
I say that Idlewild lead singer is camera shy, when I was covering a festival they were act, he just wasn't looking at the camera that were pointing at him, I am not a performer (musician) myself, and I am deadly against photos of myself being taken. I just shudder