I will announce the beta here when it is ready. Still a lot of work to finish for that though. There should be no effect on the vast majority of plugins.
However, you can test the main feature of 3.0 -- the new content server using a normal calibre install, by running
calibre-debug.exe --new-server -- /path/to/some/calibre/library/folder
And then go to
http://localhost:8080 in some *modern* browser. The new server already has more functionality than the old one, but the major new component, the in browser viewer is under heavy development right now.