SELECT DISTINCT a.geom
FROM polygon_table a, polygon_table t1, polygon_table t2, polygon_table t3
WHERE a.id <> t1.id
AND a.id <> t2.id
AND a.id <> t3.id
AND t1.id <> t2.id
AND t1.id <> t3.id
AND t2.id <> t3.id
AND ST_TOUCHES(a.geom, t1.geom)  = 't'
AND ST_TOUCHES(a.geom, t2.geom)  = 't'
AND ST_TOUCHES(a.geom, t3.geom)  = 't'