Right now your $elements var just contains one element: the process element. What you want is for it to contain all children of that element, so that should be bpmn2:process/*
Then your $e variable also needs to change a bit (see below) and the // in you're last line should the .// because you want it to be evaluated in the context of the $elements variable.
The final query should be something like this:
declare namespace bpmn2="http://www.omg.org/spec/BPMN/20100524/MODEL";
let $elements := doc('file:///C:\..\process3.txt')/bpmn2:definitions/bpmn2:process/*,
$e := $elements[self::bpmn2:startEvent]
return $elements[.//bpmn2:incoming/node() = $e//bpmn2:outgoing]