The "sqlite3" command:

Usage: fossil sqlite3 ?DATABASE? ?OPTIONS?

Run the standalone sqlite3 command-line shell on DATABASE with OPTIONS.
If DATABASE is omitted, then the repository that serves the working
directory is opened.

WARNING:  Careless use of this command can corrupt a Fossil repository
in ways that are unrecoverable.  Be sure you know what you are doing before
running any SQL commands that modifies the repository database.