If the thought of getting into a cold bath makes you sweat fill it first with.
How to keep the third floor cool.
Wearing lightweight loose fitting clothing taking a cool bath or a cold shower.
This will keep the sun from heating the top floor even more.
Not if the thermostat is on the 3rd floor thus making the ac run constantly because it is hot up there so yes keep it cool so that your 1st floor is not freezing because some dumbass put a thermostat on the top floor.
Or use a window or portable air conditioner.
Sip ice drinks and keep cool clothes to strong pulsed areas like neck and wrists that will make you cool and comfortable in this summer.
Placing a cold wet washcloth on your neck or wrists drinking something ice cold to stay hydrated using mint or menthol.
The floor of the 3rd floor room is likely totally connected to the attic without any air seal or insulation separating the 3rd floor room with the attic.
Keep a cool bottle just beside your bed and drink it in the night whenever you will feel warm and humid.
Keep the blinds curtains upstairs closed or at least close the blinds on the side of the house the sun is shining on.
Be sure not to close off too much downstairs as you will risk freezing up your system.
What if you want all areas heated or cooled simultaneously.
The most ideal option would be to look into a ductless hvac system.
Extend ductwork up there to connect the central ac.
By splitting to force air to the third floor you are removing from another area.
Place a large fan at the base of the stairs the floor below main floor and position it to blow air up the stairs.
Unfortunately none of these are ideal.
This relatively new technology is extremely quiet comfortable and energy efficient.
If you get some exterior roller window shutters for your top floor and keep them closed during the day to block the sun that might help to keep it cooler upstairs.
An answer without a question is meaningless.
Try to keep yourself cool inside your house and this is not at all a bad idea.
I would recommend adding a small additional system for the third level.
Old ways of cooling a third floor for the most part people have had two options to cool their third floor.
Central air for a third floor first of all adding new ductwork to your home is expensive.
Consider having yours insulated or replaced with sealed double insulated windows that have special layers to block much of the heat gain that comes in through windows.
Your first step should be to get that 3rd floor room better air sealed and better insulated and go from there.