Add values from two aggregate colums

Summary:

In a report where two columns are [Total B] = sum_distinct(B.score, by A.ID) and [Total C] = sum_distinct(C.score, A.ID), how do I create a third column that holds the value of Total B + Total C?

Content (please ensure you mask any confidential information):

I have three related tables, A 1-m to B 1-M to C.

I have successfully used the sum_distinct to sum the scores from B and C by A.ID into two different columns. However, nothing I've tried will provide a value for Sum B score + Sum C score by A.ID. I haven't found any documentation about how to do this either.