Lightroom wouldn't switch the location of the Catalog if the main drive wasn't connected. How could it? If the main drive where the catalog was usually kept wasn't connected you'd get a message saying it couldn't find it. You might then get a prompt to create a new catalog.
The only thing that might change without your knowledge is the location of the backup files but only if the different backup drives had the same drive letter designation.
Anyway, I'm not sure there's a simple solution but to start with make copies of your three catalogs, name the copies so that you know where each one came from.
Having done this I'd then open the catalog on the main drive and use "Import from another Catalog" from the file menu to bring in the data and file associations from the other catalogs. But you're almost certainly going to run into some issues here if there's any confusion as to which is the newest version of the different alterations. (This is why it's important to create some Catalog copies beforehand).
Maybe there'as someone out there who's been through a similar experience?