instruction: what are the three most commonly prescribed drugs for patients who were also prescribed ferrous sulfate at the same time?
response: SELECT t3.drug FROM (SELECT t2.drug, DENSE_RANK() OVER (ORDER BY COUNT(*) DESC) AS c1 FROM (SELECT admissions.subject_id, prescriptions.startdate FROM prescriptions JOIN admissions ON prescriptions.hadm_id = admissions.hadm_id WHERE prescriptions.drug = 'ferrous sulfate') AS t1 JOIN (SELECT admissions.subject_id, prescriptions.drug, prescriptions.startdate FROM prescriptions JOIN admissions ON prescriptions.hadm_id = admissions.hadm_id) AS t2 ON t1.subject_id = t2.subject_id WHERE DATETIME(t1.startdate) = DATETIME(t2.startdate) GROUP BY t2.drug) AS t3 WHERE t3.c1 <= 3