Multiple “On My Mac” Email Archive Accounts

Somehow, I have ended up with three On My Mac folders in the Archive section of my Apple Mail.  All have the same (few hundred) emails in them. They are emails from no longer used email addresses, which I don't have access to any more and I want to keep them for reference.

I want to keep one of the folders with the emails but I’m afraid if I start deleting others I’ll end up losing all the messages.

If anyone has any knowledge in this area, I would appreciate any advice you can give me.