Are all three fields defined in the same table?
Assuming that this is the case, set up a self join relationship:
YourTable::fieldA = YourTable 2::FieldA
Then fieldC can be defined to evaluate from the context of YourTable:
Count ( YourTable 2: FieldA ) // any field from YourTable 2 that is never empty will produce the same result here
The same Count can be produced in an ExecuteSQL() function call and then no added table occurrence is needed in your relationship graph:
ExecuteSQL ( "
SELECT Count ( \"FieldA\" ) FROM \"YourTable\" WHERE \"FieldA\" = ? "
; "" ; "" ; YourTable::FieldA )