I’d be interested to see examples of projects (or template projects) that use Scala3 and Scala.js (even better if it supports cross builds, but multiple examples would be great). Mill or SBT would be fine, for instance.
Make sure it uses sbt >= 1.5.0 and sbt-scalajs >= 1.5.0 (the same version numbers here are a coincidence)
Set scalaVersion := "3.0.0-RC3" in the build
If necessary, use .cross(CrossVersion.for3Use2_13) on dependencies as appropriate (e.g., in the above template, this is necessary on the scalajs-dom dependency).