"Harlan Messinger" wrote in message
1. Partly close the incoming air vents on the middle floor and possibly
the ground floor. The theory: a relatively larger portion of cooled air
will go to the top floor.

That is the easiest, cheapest, and can be very effective. Do that first.



2. Hang a ceiling fan in the stairway between the second and third floors
to push the air upwards.

3. Hang a ceiling fan in the stairway between the second and third floors
to push the air downwards.

You already have air moving in the stairways. Take a candle and hold it
near the floor. Watch the flame as you slowly move it towards the top. It
is easier to boost the hot air moving up that to boost cold air coming down.
After you do step one, experiment with a simple box fan to see what benefits
you get.