I don't have ducks, so I can't say for certain, but I was going to suggest that the 3-inch elbows are really big. I just got three to make a feeder that my roosters can get their heads in (they don't like to stick their heads in if their combs bump), and I was thinking about your post while I was standing in Lowes with my whole hand stuck through a three-inch street-connection elbow.
Some even use 4 inch for roos with really big combs but 3 seems big too. I used 4 but mine refused to stick their heads in so i took it out but i also never took out the other feeders and it was only in there a few days so thats probably why