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.
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.
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.
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.