Addition of custom field totals

Hello there, hoping someone can advise, i've built a report using Custom fields inbuilt into our Project and task module, the three custom fields circled in yellow are showing fine in the report, but when i try to build a formula to add the total of the three fields, it shows no total at all if there is no 'Cost of Job' figure in the record, and disregards the fact that the other two fields have a valu, ion those instances where there is a 'Cost of Job' figure it adds the three figures up correctly, completely stumped as