You could try creating a formula. a+b+c. In the top left of the formula wokshop window you will see a drop down box. It has two (2) choices. Make certain the "Default Values For Nulls" is selected.
Assuming a,b,c are numbers the default value for a null will become 0. So if a & b are 1, and if c is null the formula will become 1+1+0 = 2
Hope this helps
Regards,
John W.