Create 3 nodes, and two replication sets:

For replication set #1, set up the following subscription structure:

                   1
                  / \
                 2   3

For replication set #2, set up the following (different) subscription
structure:

                   1
                    \
                     2
                      \
                       3

This ought to work; according to recent reports, it doesn't.

See:
<http://lists.slony.info/pipermail/slony1-general/2007-September/006642.html>