Your first place is your home. The second one is your workplace. What is the third?
Depending on your lifestyle, you are either spending most of your time at work or at home. This is more relevant for married people, who are most likely busy with taking care of their kids, and thus spend more time at home.
The third space, is something in-between. It could be a quick trip to coffeeshop, supermarket, bar or dine-in restaurant. It is where you can have your "Me Time". Note that this is different from "vacation", in the context that the third space is part of your daily routine.
Some men, builds "man cave" for this, some women treats their garden as their third space. Be aware of this space, and make sure you aren't invading their privacy whenever they are spending their time on this space.