Windows SDKBOOL EnumChildWindows(

HWND hWndParent, // handle to parent window
WNDENUMPROC lpEnumFunc, // pointer to callback function
LPARAM lParam // application-defined value
);


Parameters

hWndParent

Identifies the parent window whose child windows are to be enumerated.

lpEnumFunc

Points to an application-defined callback function. For more information about the callback function, see the EnumChildProc callback function.

lParam

Specifies a 32-bit, application-defined value to be passed to the callback function.
Т.е. третий параметр - значение, которое передается в функцию lpEnumFunc