=SUMPRODUCT(--(A1:A10="x"),--(B1:B10="y"),C1:C10)

SUMPRODUCT can't do whole columns, so you'll need to identify ranges.

For a discussion on what this formula is doing:
http://www.xldynamic.com/source/xld.SUMPRODUCT.html


"Holly B." wrote:

I would like to sum a range if column A = x and column B = y then sum those
numbers in range C that meet the criteria. Is this possible? SumIf only
allows for one range and criteria, how can I get around this?