SSLv3 iRule reply with nice error

SSL is a lower level protocol, so the "nice error message" which you are referring to has to be at the application layer and it will depend on the application protocl in question.

If (most typically) your application protocol is http then you could just use the CLIENTSSL_HANDSHAKE event to flag that the protocol version negotiated was SSLv3.

Then in the HTTP_REQUEST event on the 1st incoming request you can dish out or redirect (to) a nice error message page.

Note that this means that the SSLv3 should be enabled in your profile to let the handshake succeed.