Test Studio allows you to extract the value
of any element to a variable and then use it later in the test as per the requirements. In this scenario you could use the variables extracted from Box 1 and Box 2 in a coded step
to get their sum stored to a third variable. For this you will need how a variable is set in code
Then, you can use the last variable, which represents the sum, back in the test steps and data bind the verification step
to verify the value of Box 3. Please note that the variable set in code will not be present in the step property UI, but still you can use it - please see point 10. in this article
I hope the above approach will suit your needs. Of course, if you have any questions, please do not hesitate to get back to me.