SELECT count(vendorsku) FROM 
(
SELECT DISTINCT vendorsku FROM
(
(SELECT ST.vendorsku FROM `stock_supercom` ST, `stock_asi` SA WHERE ST.vendorsku = SA.vendorsku) 

UNION

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

UNION

(SELECT SA.vendorsku FROM `stock_asi` SA, `stock_ingram` SI WHERE SA.vendorsku = SI.vendorsku)
) as TT
) as FT