How to cool the top floor of a 3 storey house?

Not sure if this works on stairs but I have been able to get air moving through my house by putting a (small) fan in an open door and have it pull the cooler are into the warmer room. I imagine you can also do that in the stairwell(s).

Combine this with closing the curtains or outside sunscreens and keeping the windows closed if it is hotter outside than in. And maybe a wet cloth on the windowsill (in a plate or whatever to protect the floor and wood) for the air to feel a bit more cool. The fan trick will (also) work with the windows when the air outside is cooler than that inside.

I experience a similar problem, even though what little roof space there is, has insulation. I have two floors, with the upper rooms in the roof, and no air conditioning.

My solution is to keep all the downstairs windows closed during the day, and if I don't need light close the curtains too, for better insulation and to reflect direct sunlight. That keeps the downstairs rooms cooler than outside. If you have external shutters, close them too.

In the evening as soon as the outside temperature drops to the ground floor temperature, I open the curtains and windows. In the morning I close them before the day heats up.

But upstairs, if I have the windows closed it gets much hotter than outside. So I have the upstairs windows open to let air flow through. It then stays only a little hotter than outside, and the moving air also has a cooling effect on my body when I am upstairs.

Heat rises, so the lower floor stays cool. Those windows are closed so there is very little air circulation between the upper and lower floor.

When I lived in a four-storey house (three floors plus basement) I did just the same: during the day I would keep all the windows closed except on the top floor. The lower the floor, the cooler it was. So I would say it is better for you not to open the middle floor windows, except at night.

I assume that the top floor is directly under the roof, probably with (double) tilted ceiling - following the shape of the roof.

If that is the case, then the best thing you can do is to invest in a very good thermal insulation of the roof. You will thank yourself for this both during the summer, and during the winter.

And if you want to do a proper job (and you should, especially that you do it for yourself), keep investing and thermally insulate your entire house. Discuss the topic with a specialized construction company, which understands the issue.

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Regardless of other construction factors, the only way to actually reduce the temperature to acceptable levels (additionally to good thermal insulation) is to install active air conditioning units. Discuss this topic with the construction company I mentioned above. A company specialized in HVAC systems will have to be involved also.

At the disadvantage of higher initial costs, it might be more recommended to install a geothermal HVAC - it means that the external unit is actually buried under-ground. Discuss this alternative with the specialists. Also, the HVAC system should work in dual-mode: make cold in the summer and make warm in the winter - for a better use of your investment.

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Anything else that you will try are just short-term emergency hacks, which will not make things significantly more bearable. Overall, you might even make things worse.

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The only hack which has a small chance to work is to keep the windows open during the night - to cool the air inside a little, and keep them closed during the day - to keep the heat out. But if the thermal insulation of the house is (mostly) non-existent, this will only help during the morning hours, while the sun does not yet reach hot levels.