Kind of sounds to me like they got into something that may have poison them. Can you trace back to see if there has been any changes where they normally reside? If you have a run, I would place them in there, until you can figure out what is different. 3 healthy ducks in a row is not a normal thing. I hope no more die on you, I know how devastating it is. By the way I have hundreds of sparrows and they drink the same water as my chickens and eat from the same feeder, but I have not experience anything like this. Check for any unsuspecting object in your yard. I found in mine squash dropped by a flying bird, and my chickens were pecking at it. I had a chicken get sick and died after a couple of weeks. Whether it was the squash dropped in the yard, I don't know. I took her to the vet and they said, she was fighting an infection. It was a pretty penny, but I ruled out anything that could spread and killed the other chickens.
Sometimes flying birds, especially big ones that eat dead things, carry bacteria in their beak and when they drop what they have on their beak, that is also full of bacteria. It is the only advice I can give you. Sorry I could not be of more help.
