Progress 91d and calculating a null fields

Is this for a formula in a Crystal report??

Claudia Stone
Aspacia Systems Inc

[Non-text portions of this message have been removed]
I need to perform a calculation that takes the values from 3 columns
and adds them together. Every column has the potential to contain a
null value. How do I get Progress to consider that null value to be a
zero value?

Tony Sheehan