if the value in column 3 are always numeric and there is a unique entry for each
combination (or you want to sum the values in column 3), you could use:

=sumproduct(--(Othersheet!a1:a10=x1),
           --(Othersheet!b1:b10=y1),
              Othersheet!c1:c10)

Where Othersheet!A1:C10 contains the table
and
x1 and y1 of the current sheet contains the values to match.

Adjust the ranges to match--but you can't use whole columns (except in xl2007).

=sumproduct() likes to work with numbers.  The -- stuff changes trues and falses
to 1's and 0's.

Bob Phillips explains =sumproduct() in much more detail here:
http://www.xldynamic.com/source/xld.SUMPRODUCT.html

And J.E. McGimpsey has some notes at:
http://mcgimpsey.com/excel/formulae/doubleneg.html

If the value in the 3rd column can be text, then this won't work.

But this will:

Saved from a previous post:

If you want exact matches for just two columns (and return a value from a
third), you could use:

=index(othersheet!$c$1:$c$100,
  match(1,(a2=othersheet!$a$1:$a$100)
         *(b2=othersheet!$b$1:$b$100),0))

(all in one cell)

This is an array formula.  Hit ctrl-shift-enter instead of enter.  If you do it
correctly, excel will wrap curly brackets {} around your formula.  (don't type
them yourself.)

Adjust the range to match--but you can only use the whole column in xl2007.

This returns the value in othersheet column C when column A and B (of
othersheet) match A2 and B2 of the sheet with the formula.

And you can add more conditions by just adding more stuff to that product
portion of the formula:

=index(othersheet!$d$1:$d$100,
  match(1,(a2=othersheet!$a$1:$a$100)
         *(b2=othersheet!$b$1:$b$100)
         *(c2=othersheet!$c$1:$c$100),0))