Master Flow 4 in. to 3 in. Round Reducer-R4X3 - The Home Depot
I only had to cut 1-1/2 inch of the rigid 3" exhaust line. It dry fit snug to the outside of the 3-inch exhaust. The problem was the 4-inch end of the adapter is exactly the same diameter as the 4-inch exhaust cowling coming off the fan. For a DIY market, that is not helpful. I used a No Hub Coupling to make the connection work.