I started by wiring a ecobee smart thermostat to my 30year old furnace. That cut the power use by half. Sensors on each floor and the master improved it Controlling the bathroom fans and using them to vent heat helped too.

Adding the inline fan pushed cold air from the lower level up to the 3rd floor, normalizing a 4C temp differential in about 2 hrs . Slight fan noise but nothing like an AC. 1 small AC on the upper level held down the whole 2300 sqft house