I am trying to total 3 different Running totals. Is this possible?

Here is an example of what I am trying to do. Let's say that there is a grouping on a job no 100 where I am totaling parts for that job. I have a running total on Part X, Part Y, Part Z. Is there a way to add up all of these parts? My problem is when one part isn't used for that job. It doesn't give me a total then.