what are the three most frequent medications that were prescribed during the same month to patients aged 30s after having been diagnosed with nb obsrv suspct infect, in the previous year?
select t3.drug from ( select t2.drug, dense_rank() over ( order by count(*) desc ) as c1 from ( select admissions.subject_id, diagnoses_icd.charttime from diagnoses_icd join admissions on diagnoses_icd.hadm_id = admissions.hadm_id where diagnoses_icd.icd9_code = ( select d_icd_diagnoses.icd9_code from d_icd_diagnoses where d_icd_diagnoses.short_title = 'nb obsrv suspct infect' ) and datetime(diagnoses_icd.charttime,'start of year') = datetime(current_time,'start of year','-1 year') ) as t1 join ( select admissions.subject_id, prescriptions.drug, prescriptions.startdate from prescriptions join admissions on prescriptions.hadm_id = admissions.hadm_id where admissions.age between 30 and 39 and datetime(prescriptions.startdate,'start of year') = datetime(current_time,'start of year','-1 year') ) as t2 on t1.subject_id = t2.subject_id where t1.charttime < t2.startdate and datetime(t1.charttime,'start of month') = datetime(t2.startdate,'start of month') group by t2.drug ) as t3 where t3.c1 <= 3