how many patients since 3 years ago received a rapid respiratory viral antigen test microbiology test?
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select microbiologyevents.hadm_id from microbiologyevents where microbiologyevents.spec_type_desc = 'rapid respiratory viral antigen test' and datetime(microbiologyevents.charttime) >= datetime(current_time,'-3 year') )