How about this? Although i agree query might get longer when there would be more than 3 items

SELECT DISTINCT CustomerID

FROM #Purchase P

WHERE EXISTS (SELECT 1 FROM #Purchase P1

WHERE P.customerid = P1.customerid

AND P1.productcode = 'A'

)

AND EXISTS (SELECT 1 FROM #Purchase P2

WHERE P.customerid = P2.customerid

AND P2.productcode = 'B'

)

AND NOT EXISTS (SELECT 1 FROM #Purchase P3

WHERE P.customerid = P3.customerid

AND P3.productcode = 'C'

)