Another way of adding class="active" is to just add an if statement with If snippet, or I guess, even an output modifier.
With If snippet in the template:
<li><a [[If? &subject=`[[+id]]` &operand=`[[*id]]` &then=`class="active" `]]href="[[~[[+id]]]]" title="[[+pagetitle]]">[[+pagetitle]]</a></li>
Could also be used to make another getResources call I guess... not tested that though, by adding a similar If call with
&then=`<UL>[[getResources? &tpl=`anotherTplWithPerhapsYetAnotherGetRes-callForThirdLevel` &parents=`[[*id]]` ]]`
Although I might put the "IFs" in a sepetare chunk...
Perhaps not the prettiest way, but as usual with MODx you can do anything, and in a huge number of ways...