Can I share an already prepared album?

Does the order of the photos in the shared album matter? If not, do as Tony suggested and select all at once and drag them to the album all at once. But then the photos may be rearranged when added to the album. Only by adding one at a time, you can achieve the same order of the photos in the shared album as in your original album.

If you want to ensure the correct sequence and add the photos one at a time, be considerate, and do this before you share the album. Wait, until all photos have been added, before you share the album. Otherwise the subscribers may receive an alert for each photo you add to the album, which might be annoying for 3000 photos.

Shared albums are separate from normal albums. You can't share normal albums, but it is easy to create a new shared album, then drag the photos from your prepared album into the shared album.

You should be able to select all the images in the prepared album (CMD-A) and drag them in one step to the shared album.