You are, of course, welcome to build your own version of the SQLite library or shell with the [SHA3 extension](https://sqlite.org/src/file?name=ext/misc/shathree.c&ci=tip) built in. The reason that all the sometimes useful extensions are optional, rather than forced into every build, is because the 'Lite' part of SQLite is a real feature, taken seriously by the developers and appreciated by many clients who target smaller platforms. When incorporated into a build instead of being a separate dynamically linked library, the extension code acts in nearly all respects just like built-in functions do.