SELECT count(S1.vendorsku) FROM 

(SELECT * FROM `stock_supercom` ST, `stock_asi` SA WHERE ST.vendorsku = SA.vendorsku) as S1

JOIN

(SELECT * FROM `stock_supercom` ST, `stock_ingram` SI WHERE ST.vendorsku = SI.vendorsku) as S2

WHERE S1.vendorsku = S2.vendorsku