Puzzle Bobble/Bust a move

I'm making a game like that. There is a gun that shoots bubbles, and when there is 3 or more same kind of bubbles colliding with each other, they disappear. That's what I'd like to do, but I don't know how to tell the game when there is 3 or more bubbles colliding with each other, and how to destroy them. It always destroys just 1 bubble per shot.

Hopefully someone undesrtood, even though my english is bad...

You could look at the Brix part of the Advanced Gameboard example.