If you are really short of time, this is what you will do.
1. Create a script-based Calculated Value provider, and fill it in with the sample code.
2. Replace the sample code with the logic of reading the three integer attributes Value1, Value2 and Value3, add them together and return the total.
3. Attach the provider to attribute Total.
4. Set the attribute Total to depend on attributes Value1, Value2 and Value3, so the provider will be triggered when any of the three attributes is changed.

You can find most of the stuff here:
https://jazz.net/wiki/bin/view/Main/AttributeCustomization#Script_based_calculated_values

The total number will be "displayed" straightaway when you change any Value# value, but will only persist after a Save.

See https://jazz.net/library/article/1093 for hints how to do this in JavaScript.

I had to use parseInt See https://stackoverflow.com/questions/460172/how-do-i-add-an-integer-value-with-javascript-jquery-to-a-value-thats-returni to make that work.

Also see https://jazz.net/forum/questions/78641/simple-javascript-for-calculated-value-of-3-other-values-in-rtc-work-ticket/78655

or

https://jazz.net/forum/questions/171738/how-do-i-calculate-custom-decimal-attributes-that-represent-dollars-in-rtc

for other hints.

In Java a solution approach is described here: https://rsjazz.wordpress.com/2012/07/31/rtc-update-parent-duration-estimation-and-effort-participant/ in your scenaro you would not have to traverse a link though.