How to add only certain columns in a matrix total

[Jordancdc]

I have a matrix which reads a stored procedure. The stored procedure produces multiple columns, some of which are subtotals. I would like to only include 3 of the columns in the grand total column. How do I create a column of only the sum of certain columns?
Sorry if this is confusing. I'm in a rush and hopefully there is a simple way to do the code. BTW this is using SQL Server Reporting Services 2008.