1) ISPConfig 3 is stable.
2) The plugins will not work. ISPConfig 3 does not have this plugin interface as you can use the pacakges of your linux distribution instead.
3) There is no way to update a ispconfig 2 system directly to ispconfig 3.
But if you are satisfied with the functions of ispconfig 2, there is no need to update to ispconfig 3. ISPConfig 3 is a multi server controlpanel, so if you just manage one server, you can stay with ispconfig 2.