What is the total units of products sold on the day with the highest max temperature in store no.3 in 2012?
highest max temperature refers to Max(tmax); store no.3 refers to store_nbr = 3; in 2012 refers to substring (date, 1, 4) = '2012'; total units refers to sum(units)
SELECT SUM(units) 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 T1.`date` LIKE '%2012%' GROUP BY T3.tmax ORDER BY T3.tmax DESC LIMIT 1