Is there an http proxy server configured in Nexus Repo 2? If there is, you will need to add the host name of Nexus Repo 2 to the non-proxy host list. The reason for this is that the http proxy settings will get synchronized from 2 to 3, and when this happens the Nexus Repo 2.x host name will need to be on the non-proxy host list, otherwise outbound communication from 3 to 2 will start going through your http proxy server.
Rich