Have you added the X3 wsdl as a web service reference to your .net application?  That gives you the wrapper for the SOAP web service calls.  As far as serializing and deserializing, I normally create classes representing the request for each web service, and then use Newtonsoft Json to serialize and deserialize.  I'm sure you could create a generate function to do the serialize/deserialize, but I normally do it within each function I am calling. It is also much easier if you return the inputXML results from X3 in json format, if you aren't already doing that.

In VS you can paste XML/JSON as classes. You can use postman, curl, etc to get an end point result, then copy/paste that result into VS. When you paste, right click and select 'xml as class'. You are then free to change those class names as you see fit as the default names are generic.