Adobe Desinger: Caluculation with JavaScript

Hello collegues,

I don't know whether this is the right place for my question but I will try it. I created an Adobe Form with the Adobe Designer. I would like to calculate the sum of three fields and display the result in another field. I have to use JaveScript but I am not an expert in this topic. Very helpful would be a litte example from with a simple addition of two fields.

Thanks in advance,

Alex