PREFIX : <http://example/>
PREFIX apf:     <http://jena.hpl.hp.com/ARQ/property#>

SELECT DISTINCT ?x
WHERE { 
  ?x :p ?v1 .
  ?x :p ?v2 .
  FILTER(?v1 != ?v2)
  OPTIONAL{?x :p ?v3 . FILTER(?v3 != ?v1 && ?v3 != ?v2)}
  FILTER(!bound(?v3))
}