One of the most important patterns in third places is that the third place must define and delimit the zone of the activity that is the purported purpose of that space. There must be a place to serve drinks / lift weights / browse books as well as a space to *not* do those things.
Activity can be constituted in such a way that it anticipates its reception, and a functional space can be constituted in such a way that it organizes the space to receive that function.
Sometimes you'll notice that people will subvert the intended organization of a place of commerce to create a space for reception. For example, even in crowded bookstores people will collectively shape little wedges to stand apart and take in something, or generate little wedges on which readers might perch themselves.