One limitation in Radmin 3.0 beta (and 2.0) is when
connecting through another middle host, it only asks
for one login / password, and when the destination machine
has a different login or password, the connection fails
with "unknown error".
I don't know if "connect through" is fully functional in the new beta 3.0, but in the old beta and 2.2 when you connect through a host you have to put in the password of the host, THEN you will get another prompt for the remote machine.
I also do not believe that this feature is not working in this beta release. Even with the same username and password on both host and remote machines I am still getting the same "unkown error" that David is experiencing. No such problems using v2.2