Avoid using a 3" duct as this creates very high static pressure, reduces airflow dramatically, and over the long term, reduces motor life in the fan. If you are remodelling a house with existing 3" duct runs, one trick is to use the 3" as a guide for the 4" replacement duct. Simply slip the new 4" duct over the 3" and push it to the fan or to the wall to avoid having to expose the entire duct. Once the new duct is in place, pull out the 3" and dispose of it.