What was the dew point on the day the most units of item no.5 were sold in store no.3 in 2012?
item no. 5 refers to item_nbr = 5; store no.3 refers to store_nbr = 3; in 2012 refers to SUBSTR(date, 1, 4) = '2012': most units sold refers to Max(units)
SELECT dewpoint FROM sales_in_weather AS T1 INNER JOIN relation AS T2 ON T1.store_nbr = T2.store_nbr INNER JOIN weather AS T3 ON T2.station_nbr = T3.station_nbr WHERE T2.store_nbr = 3 AND SUBSTR(T1.`date`, 1, 4) = '2012' AND T1.item_nbr = 5 ORDER BY units DESC LIMIT 1