how to calculate the sum of multiple fields for conditionals?

You can follow this guide on how to use conditional logic in your form: http://www.jotform.com/help/57-Smart-Forms-Using-Conditional-Logic.

If you wish to perform a calculation, just select "Update/calculate a field".

You'll need t set an "IF" statement on when the calculation will happen:

You can also perform a form calculation using the form calculation widget. Here's a guide to learn more about it: http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form 

Let us know if that works for you.