Hi,
Not sure if I understand your question fully, but the following would give
you the answer you want (15) from the sample data:

=SUMPRODUCT(--(A1:A3=B1:B3)*C1:C3)

Change A1:A3, B1:B3 and C1:C3 to suit your actual data.
Note that SUMPRODUCT requires all columns to be of equal length.
Regards - Dave.