(was tempted to stop there as that's all you asked...)
An ExecuteSQL query comes to mind as a way to get this count with a WHERE clause that omits records from the client count when related Product counts have one of these other values, but perhaps this approach is simpler.
I am assuming this relationship:
Clients::__pkClientID = Products::_fkClientID
(this is the wrong relationship, BTW, if the same product can be linked to more than one client)
In the Clients table, you can define a calculation field like this:
IsEmpty ( FilterValues ( LIst ( "B" ; "C" ) ; List ( Products::Status ) ) )
This value will be 1 (True) only if status "B" or status "C" is NOT specified for at least one related products record. Otherwise, it is zero (False).
With this in place, you can perform a find with two requests on the products layout. The first request specifies status "A" in the Products table and the second request would be an Omit request specifying 0 in this new calculation field in the clients table.
Thanks for the information.
The Product only ever is connected to one Client so I believe this relationship is fine, but if you disagree please let me know.