Adrian: you should be able to buy a 4" to 3" reducer that would allow you to connect a new fan into the old "ducts". The volume of air moved by bath exhaust fans is so low that the smaller piping should not have any effect on the fan's efficiency...................
: My townhouse bathrooms have the original, low-quality exhaust fans that appear to vent into 3-inch ducts. I want to replace these with quieter, more efficient units. Changing the ducting to 4-inch is nearly impossible. What fans will work best pushing into a 3-inch duct? Should I add a second fan in the master bath (the one there is in a separate water closet)?