Try

=IF(ROUND(A3-SUM(B3:C3),2)=0,"-","check")

Change 2 to the precision you want.

The problem is the way that Excel represents values internally: 64-bit-binary floating-point. Values are represented by the sum of 53 consecutive powers of 2 ("bits") times an exponential factor. Consequently, most decimal fractions cannot be represented exactly.

If you really want to study this, see:

http://faculty.tarleton.edu/agapie/d...atingPoint.pdf