Can't pick fields for a sum calculation

I'm trying to do a very simple calculation, adding 3 fields together using the sum function in a fourth field. When I go edit the property of the field with the calculation in it, I am checking Value is the sum of the following fields and all I can do is select or deselect all of the fields. I need to just sum 3 of the fields and save the total in a forth field formatted to surpress the zeros. 

Thanks for the help!!