Mozilla Firefox
Firefox Nightly supports HTTP/3. The experimental Firefox Nightly builds are released, well, nightly. They may have some stability issues that you should keep in mind when testing. You can install it alongside regular Firefox; the Firefox Nightly installer can be found here.
Running
Firefox's HTTP/3 is not enabled by default. It can be enabled by toggling the network.http.http3.enabled value to true in about:config,