Will a 4“, 100CFM bathroom exhaust fan work with 3” exhaust duct

Some furnaces have more than one breaker

Well, most honestly.

• The Air Conditioning unit has a breaker
• If present, electric heating elements (e.g. emergency heat) have one or more breakers
• The blower will have its own breaker, typically shared with the electronics on a gas furnace
• A built-in humidifier or dehumidifer may have its own breaker
• Depending on location, the 24V thermostat transformer may be on yet another breaker

Unrelated, the radon venting system has its own breaker.

One way to cut away the excess metal would be to use a rotary Dremel type tool with abrasive cutoff wheels. This method allows the metal to be removed without bending and deformation that other cutting methods would typically cause.

If you go this route be prepared to use up a good sized quantity of the cutting wheels. When I am using my rotary tool and have a large amount of cutting to do I have about ten of the mandrels that hold the cutting wheels preloaded with cutting disks. This way when one wears down or breaks I can quick load another mandrel into the chuck on the rotary tool and continue cutting.