It really depends on the position. I once traded my Queen for 3 center pawns, and a rook, with a great position to boot. My opponent was so flustered, he lost the game.
A queen or 3 minor pieces

I would think the 3 minor pieces would tend to be better as long as they are coordinated. If you have the queen, then you need to do something good before your opponent can coordinate his pieces.

I'd say the three minor pieces. There are probably more tactics that you can achieve with the three minors rather than a queen.

I can't recall a situation on the board where a Queen proved to be superior to three minor pieces ...

Interesting thread, I just watched a video by Yasser Seirawan on this very topic.
He says stop looking at what is off the board ie counting captured material & look at what is on the board. He then demonstrates several games where a Queen sacrifice was valid & where it was not
Have a look at the video, its top rate analysis
http://www.youtube.com/watch?v=0pWqTfnKbH8
It's not an easy thing to achieve, in a fairly level game, exchanging your Q for 3 pieces....`
It happens mainly in some openings variation. But otherwise it is really hard I agree.
And yes, it depends on the position as always.
You may want to watch this video, it is about this topic. http://www.youtube.com/watch?v=0pWqTfnKbH8
Which would you prefer to have? In positions where there are a lot of pieces still on the board I'd rather have the three minor pieces as they can harass the queen and the queen can't exchange itself for anything on the board (except for two rooks of course). In positions with a lot of the space I'd take the queen as the pieces are pretty much bound to the defense of each other or the queen will gobble them up.