Is that three copies in Three folders?

No, three copies in sam folder, INBOX.

I tried reinstalling Thunderbid, but three copies still come up

did you try repairing the folder? Right click, select properties and then the repair button.

yes but every time i get a message The operation failed because another operation is using the folder. Wait for the operation to complete and then try again.

Try this procedure and see if it makes things better. https://support.mozilla.org/en-US/kb/rebuilding-global-database