Among the stations with 3 stores, how many stations have a station pressure of no more than 30 on February 18, 2014?
station with 3 stores refers to station_nbr where Count(store_nbr) = 3; station pressure of no more than 30 refers to stnpressure < 30; On February 18, 2014 refers to date = '2014-02-18'
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 )