Kim,
Actually it's not 3 minutes it's 3 1/2 minutes. But I usually say three minutes because it's human nature to over run and 4 minutes is too long.

Now of course in the millions of songs that have ever been written you're going to be able to find a few exceptions. But I'm not a good enough songwriter to write those one-in-a-million songs.

It's also a function of MODERN listeners, in times gone by, people had more time - and songs stories and books were all longer.

Here's an exercise for those who don't believe me.
Next time you hear someone singing a long song - take a good look at the audience. See how many are paying attention. See how many are waiting for the singer to get to the point. See how many are just waiting for the song to end.

You can do this yourself by singing something like "The Barley Mow"

Bert.