In businesses with a category of food, how many of them have a star rating below 3?
category of food refers to category_name = 'Food'; star rating below 3 refers to stars < 3
SELECT COUNT(DISTINCT T1.business_id) FROM Business AS T1 INNER JOIN Business_Categories AS T2 ON T1.business_id = T2.business_id INNER JOIN Categories AS T3 ON T2.category_id = T3.category_id WHERE T3.category_name = 'Food' AND T1.stars < 3