EllieB,
It's a little more complex, but not by much.
Overview: I would suggest creating a new (4th) library. Then load each of the first three libraries into calibre, copy the books into the 4th library with merge books enabled. Use the Find Duplicates plugin to find any stragglers. Then run the Quality Check plugin to get everything well organized. After that you could send the 4th library to DropBox and after a bit of time to make sure you haven't goofed anywhere, delete libraries 1-3.
If you have any custom columns that need to be included in the new library, remember to load that library and check Copy Structure. If you've got custom columns that need to be added from multiple libraries, copy one library's structure and then add the remaining columns manually. Only after all of the columns are present should you continue to copying all of the books into this new 4th library.
As for copying the books, that is done by loading one of your old libraries and then:
- Highlight all the books (click on one book, press CTRL+A)
- Right click any highlighted book
- Copy to Library (NOT: Delete after Copy!)
- Select 4th library name
- Be patient while the books are copied - it can take awhile if you have lots of books.
You will need to have previously set the Merge options (Edit Metadata>Merge Book Records... hmm, I think there is another set of Merge options somewhere else also.)
Let me know if you need more info/details.