I have 3 Photos Librarys. How can I combine them?

You can import the photos directly from one photos library into another library to merge one library into another, as described here: Import photos from another library in Photos on Mac – Apple Support (UK)

But note, this will only import the photos and videos, not the albums, and not the keywords.But Photos will scan for duplicates and avoid duplicates on import.

You will get a much better result by merging your libraries in iCloud Photos. when you upload each of your libraries into iCloud Photos in turn, Photos will merge your libraries in iCloud, preserving all metadata and albums. But it will take much longer, if your libraries are large.

How large are your libraries? Photos is doing a lot of background processing for all photos and videos. If you let your libraries become very large, you will have long periods of Photos being slow because of the scanning. I am trying to keep ma libraries smaller than 50000 items and are keeping only my favorite photos and videos in my system photos library.