You really don't want to use entire columns in a SUMPRODUCT formula so you'll need to limit the rows:
=SUMPRODUCT('C:\path to file\[WORKBOOK.XLSX]Dec15 Pt2'!$S$1:$S$1000,('C:\path to file\[WORKBOOK.XLSX]Dec15 Pt2'!$N$1:$N$1000=$A3)*('C:\path to file\[WORKBOOK.XLSX]Dec15 Pt2'!$R$1:$R$1000=$B3))
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