instruction: what were the three most frequent lab tests?
response: 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 GROUP BY labevents.itemid) AS t1 WHERE t1.c1 <= 3)