Having next ^3.0.0-beta or even next ^3.0.0-beta16 in my package.json or direct npm install 'next@^3.0.0-beta16' resolves to 3.0.0-beta9 (both npm and yarn behaves the same).
npm install 'next@beta' works fine, though.
This issue was also reported by @gragland in #2147 (comment)
I am not sure if this is Next.js issue, but 2.x-beta releases worked as expected.