You gave the relay name "unkown"?
Or no name at all? There has been no change in v3.0 in that functionality, so make sure you have given the relay a name and also that it isn't controlled by something else.
You could also try entering:
http://[url]/relaycontrol/on/4
and
http://[url]/relaycontrol/off/4
to find out it the relay works then.