Hi,
you may write the metadata to the images. Then you can remove the three collections and create the one you like.
1. Make sure to write ALL wanted metadata from the database to the images:
1.1. Go to Tools -> Maintenance
1.2. In the "Common options", select "Whole albums collection".
1.3. In the section "Sync Metadata and Database", select "From database to image data".
1.4. Press the "Settings..." button to see which data is written to the metadata and adjust to your needs.
1.5. Click "OK" to run the sync.
2. Remove your three collections.
3. Add the root folder for your desired one collection.
3.1. All metadata which was written in step 1 is read from the images and added to the database again.
Regards
Eddie