I'm gonna guess that the leak in your master bath is from your bath exhaust ducting, that's what the single duct looks like it could be. You'd need to trace back the other two ducts to see what they go to, probably a different bathroom. The vent on the outside of the roof could be leaking and water is running down the ducting and dropping off onto the ceiling. It could need to be caulked or it could of been installed wrong and the shingles don't overlap the vent flange, no way of knowing without seeing. Here's a link that shows how to install one, perhaps it will help in your repair.
http://www.homestore.com/HomeGarden/...?poe=homestore
If you live in a cold climate then those ducts should also be insulated to prevent condensation from forming and running back down into the fan in the winter time. Here's another link that'll show you how to insulate them.
http://www.askthebuilder.com/546_Bat...tilation.shtml
To fix the leak at the roof vent shouldn't cost too much, call an experianced roofer in your area for the repair.