You can create 3 report variable for the formula of column A,B and C and then you can apply above formula given by me.
Create variable-
A=
If (condition) then "X"
elseif (condition) then "Y"
else Measure object
B=Similar to A
C=Similar to A
Then use this formula to calculate sum-
=If IsError(ToNumber([Column A]))=0 And IsError(ToNumber([Column B]))=0 And IsError(ToNumber([Column C]))=0Then ToNumber([Column A])+ToNumber([Column B])+ToNumber([Column C])
~Anuj