Question: Among the stations with 3 stores, how many stations have a station pressure of no more than 30 on February 18, 2014?
SQL: SELECT COUNT(station_nbr) FROM weather WHERE `date` = '2014-02-18' AND stnpressure < 30 AND station_nbr IN ( SELECT station_nbr FROM relation GROUP BY station_nbr HAVING COUNT(store_nbr) = 3 )