###HUMAN: what are the top three most common specimen tests performed since 2 years ago?, ###ASSISTANT: SELECT t1.spec_type_desc FROM (SELECT microbiologyevents.spec_type_desc, DENSE_RANK() OVER (ORDER BY COUNT(*) DESC) AS c1 FROM microbiologyevents WHERE DATETIME(microbiologyevents.charttime) >= DATETIME(CURRENT_TIME(), '-2 year') GROUP BY microbiologyevents.spec_type_desc) AS t1 WHERE t1.c1 <= 3