Ok, I have found the solution by myself a little later after I posted the first question. I'll provide a basic description in case someone needs this in the future.
I put 3 example courses into a separate hidden category and allowed guests to see them (courses are in hidden category, so unless you have a link you cannot access them, because they're not in any listings). I can then link these from my main course and users enrolled in main course are able to access them.
If you want another (weak) layer of protection, you can setup a password for guests to access and provide this password only in the main course.
Martin