Thank you for your question on the model F770 exhaust fan. The F770 is the finish pack of 4 units (normally purchased by contractors). A single unit is just the model 770 and it is designed to use 4” duct only. We do not recommend using anything smaller than 4” duct with this model. You may want to google model EZ80N which is a unit designed for easy retrofit applications and it comes with a 4" to 3" adapter.