what was the three most frequently taken test since 6 years ago for patients with age 20s?
select d_labitems.label from d_labitems where d_labitems.itemid in ( select t1.itemid from ( select labevents.itemid, dense_rank() over ( order by count(*) desc ) as c1 from labevents where labevents.hadm_id in ( select admissions.hadm_id from admissions where admissions.age between 20 and 29 ) and datetime(labevents.charttime) >= datetime(current_time,'-6 year') group by labevents.itemid ) as t1 where t1.c1 <= 3 )