sum is intended for use when you are adding up a lookup or link field. If your three columns are just normal number columns then you should simple write:
Ah, that is rather surprising I agree. I’ve made this issue to make numeric operators treat empty as 0 as I think that is much more sensible. This is an unintended consequence of the way the database Baserow uses treats empty values, any empty value + any other numeric value will always equal empty.
For now you could also use the when_empty function like this: