in react-router v4 relative paths are supported. The history library resolves relative paths ‘just like a browser would’ so that a
<Link to="two" />
or a
history.push({ pathname: '..', search: 'foo=bar' });
at the url site.com/path/one will redirect to site.com/path/two and site.com/three?foo=bar.
However NavLink does not work with relative pathnames because it doesn’t resolve it’s own relative path (which is nontrivial to do). However there are some community packages for this, but I haven’t tested them.
Alternatively, and this is what I do, you can get a match object from your parent Route (or from the context router.route.match) and use it to build your relative to in NavLink:
<NavLink to={`${match.url}/three`} />
this way your component can work independent of it’s parent Route(s).