Three photos libraries, which library can I delete?
You need to be careful here…

The old iPhoto library doesn't take up any actual space on the drive. When it was first translated to the new Photos structure all that actually happened was that hard links were made. These are multiple pointers to the same file. Only when both pointers are removed, eg by deleting the same file from both the old &amp; the new library, is the file itself actually deleted.

There's a further issue though, &amp; I'm not sure how this is dealt with on OSes since High Sierra.
APFS can't use hard links.
I really don't know what this means for the two sets of data previously reliant on hard links. Perhaps they are now 'real' so deleting the old iPhoto library will actually free up space.

In either case, deleting the old iPhoto library should be safe. It has all already been migrated to the new structure.

For the other two, you need to be more circumspect.  

Load up each &amp; see what's actually in there. It's quite possible to have multiple valid libraries, each with its own content. At launch, hold the  Opt ⌥  key &amp; you will be presented with a dialog allowing you to choose which library to open.

There's no way I know of to merge the two, should it be desired; the only way I can think of would be to export from one to a new folder, then re-import to the other - then you could delete the now spare library &amp; the intermediate folder.

I'd be very tempted, however, to make sure to have a good backup before you start… just in case.