Well, then you could create a custom report for XMC that implements an SQL query against the NAC end-system database. That query could select all unique switchport names (would need to combine the switch IP with the port name) and then count the occurances. That should give you the number of end-systems seen per port. Then filter for those which have a count larger than 3.
Haven't tried to solve this specific use case but with SQL it should be doable.
But this requires some knowledge of the DB and the XML report.
Unfortunately, not as easy as a FlexView...