You can still use the old method of specifying every node on each server. Server 3 needs the following set
'cluster-nodes' = [ 'serv1.xmpp-test.org' , 'serv2.xmpp-test.org' ]
Server 2 needs
'cluster-nodes' = [ 'serv1.xmpp-test.org' , 'serv3.xmpp-test.org' ]
and so on…
However, we do not recommend this.