Sorted now. Looked through the code and intro on https://github.com/pcmanus/ccm (linked to from Run multiple cassandra nodes (a cluster) from the same machine?), and that keeps all port values the same apart from that for JMX connection.
Having made those changes, plus setting the initial token on each thanks to @BryceAtNetwork23, and specifying the IPs of all 3 servers as seeds, they now form a cluster.