Untitled

(SELECT 
    product_id, 'store1' AS store, store1 AS price
FROM
    Products
WHERE
    store1 IS NOT NULL)
UNION
(SELECT 
    product_id, 'store2' AS store, store2 AS price
FROM
    Products
WHERE
    store2 IS NOT NULL)
UNION
(SELECT 
    product_id, 'store3' AS store, store3 AS price
FROM
    Products
WHERE
    store3 IS NOT NULL)