Crystal Reports Multiple if isnull then

Message

   I'm new to crystal so hopefully my question will have a simple answer!
I have created a report which has three columns (Report Fields). Lets call them {A} {B} and {C}.

I need to add a fourth column which adds the three columns up. But it won't do it if any of the three columns is blank ie(nul).  

To get around the problem of column A being null I can use

If isnull({A}) then
{B}+{C} else
{A}+ {B} +{C}.........

but this doesn't help if columns B and C are Null. I tried to use a kind of multiple 'If is null ' but no luck.

Any help much appreciated.