Third Orders are for anybody. For, say, the Dominicans, you have the friars, who are First Order, nuns are Second, Sisters are Third Order Regular (regular = with a regula, a rule), and laypeople can be Third Order, um, not regular. Third Order members live the spirit of the Carmelites, Dominicans or whatever as best they can in their state of life (praying some of the Hours, following the spirituality of the Order) and have meetings every so often with other tertiaries (Latin for “Third”). I have a friend who’s a Third Order Carmelite, he’s 24 or whatever & plans to marry someday, & have met Third Order Franciscans & Dominicans on the boards here. St. Thomas More was a Third Order Franciscan.