There's a huge difference between roots and rhizomes. Rhizomes have buds on them and can produce new plants. Roots have no buds and cannot produce new plants. The best way to learn the difference is to actually dig the whole crown up and take a look. It seems pretty harsh but it's the best way to learn and the fact that it's a 3rd year plant makes it a good candidate for this procedure. By the 3rd year, if the plant has been growing well, you'll see little if any reduction in vigor after pruning it back.
The alternative will be years of experience digging. As you dig, you'll find what looks like roots, but once you look a little closer you'll see rings of buds anywhere from 4-8 inch intervals along the structure. If you see buds, it's a rhizome. If you don't see buds, it's a root and can safely be left in the ground without worrying about new growth coming from it. I'll try to dig up some pictures when I have time.
edit: Better than pictures. I remember seeing this a few days ago. Please don't watch if you're not handy with sharp objects:
https://www.facebook.com/photo.php?v=637559506293579&set=vb.156268764422658&type=2&theater