Calculated fields; sum of three fields compared with a 4th

I have a worksheet which, among others, has 3 columns of different type changes, plus a column of the totals of those 3 columns (derived by a different methodology). I'm doing a pivot on these values, so in my VALUES section I have the 4 columns. I'd like to add a check that the 3 columns add to the value of the 4th column. I'm sure I need to do this as a calculated field, but not sure how to go about it. I've attached a little mockup of what I'm wanting to do. Any help would be appreciated.