You can. It's just that no browser that supports HTTP/3 will accept it as a legit endpoint with a valid root. So they won't connect to the HTTP/3 endpoint at all and you won't be able to access the HTTP/3 self-signed website.

And before anyone goes there, no, setting up your own root CA is not an option. Unless you get can Google/Apple/Mozilla/etc to include your root CA in their browser trust stores it doesn't help a random person visit your website at all.