When is a queen better than 3 minor pieces (or vice versa)? Obviously a lot is going to depend on the exact position, but I'm asking if there are any general signposts - e.g. bishops usually get better than knights if the position is open, or in an endgame, but I can't say the same about Q vs 3 minors.
The queen does a good job when there are a lot of weaknesses to attack, especially if the opponent's king is out in the open, so there are a lot of options for double  attacks.
The pieces are generally stronger if they can coordinate and the king is still relatively safe. Earlier in the game that's usually the case.