How to perform calculations on a form?

I want to find the sum of the results of two fields and multiply that with the result of another field. Is that possible?

Yes this should be achievable with Conditions and Calculations. You just need to apply your computation on the condition you're creating.

Here's an example form https://www.jotform.com/71436967186974 and below are the conditions that used:

Logic:

IF Number 1 field IS FILLED AND Number 2 field IS FILLED

THEN INSERT the total to Sum of 1 and 2 field

IF Sum of 1 and 2 field IS FILLED

THEN INSERT the product to Product of Total X Number 3 field

Related guides: 

Smart-Forms-Using-Conditional-Logic

Conditional-Logic-Inserts-Text-Calculation-Into-A-Field

How-to-Perform-Form-Calculation-Using-a-Widget

You may clone the form I created if you want to better understand how it was setup.