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Copy *.sqlite database to *.db3 database
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There are several ways to do what you ask.

You can open your DB with the SQLite command line shell, then use its .backup commend to write it out with whatever name you like. (Type '.help backup' in that shell to see how.)

If no process has the DB open, you can use the usual OS file copy methods to copy the original DB file to one named as you like.

Many more ways can be imagined.  These are the easiest, IMO.