You will probably need to improve those tables by adding an ID column to two of them: SQL will not allow you to have two identical rows and it's really easy to sell to of the same product on the same day to different customers for example.
But joining them is simple:
SELECT p.PID, p.PName, (pu.pqty - s.sqty) as Balance
FROM tbl_prod p
JOIN tbl_purc pu
ON p.PID = pu.PID
JOIN tbl_sold s
ON p.PID = s.PID AND s.sold_date = pu.purc_date