Cubase SX3 for Windows 10 can be downloaded at a link which was kindly sent to me by Steinberg Technical Support. (can’t put it in this message, this forum does not allow links, but you can find it in a page titled “Converting Cubase VST songs (ALL/ARR) into CPR format” in the Steinberg Support)
I need a USB dongle to use it.
Beforehand, I had tried to install Cubase SX3 on Windows 10 with the original CD in safe mode (as suggested by ZingarMusicLab) but could not do it because my keyboard and mouse don’t work in that mode.
A demo version of Cubase Elements (works for 1 month) can also open old .cpr files but the settings of your projects (delay, reverb…) are lost while they are kept when you reinstall SX3. Also the design of Elements is much different, so, in the end, in 2020 I still far prefer using SX3 the same way I did in 2004.