I just read a great article about third orders. It was in Lay Witness Magazine [LINK]

At the end of the article there is linkage to some third orders that I was not aware of. I know of the Carmelites and the Dominicans and the Franciscans, naturally. But I didn't know that the Trinitarians had a third order, and I didn't know that the Mercedarians had one either. There are more... it's a very useful article