I installed Krita 3.0 Alpha and now I can't open an older file with several layers with painting and text. Other files with several layers are ok.

Wait for the Beta to come out. We've fixed a nasty bug that crashed Krita on files with filter layers, and that fix is in the beta. It should be released coming week.

And if that still doesn't work, please open a bug report and make the file available to us -- those are the things that are going to block the release.