Change the
NOT IN
to an outer join and add the other fields to the missing conditions
SELECT P.Stockno
,P.Modelno
,P.ServiceTag
,Count(P.Stockno) as ClosingBal
FROM Table1 P
left OUTER
JOIN Table2 t2
ON p.servicetag = t2.servicetag
AND p.stockno = t2.stockno
AND p.Modelno = t2.Modelno
WHERE t2.servicetag IS NULL
AND t2.stockno IS NULL
AND t2.Modelno IS NULL
GROUP BY P.Stockno,P.Modelno,P.ServiceTag