You don't need to spend all your time somewhere for it to become a decent third space. You just need to go regularly, at a similar enough time that you start building rapport with the people who also go at the same time. If you go to the gym or bar for an hour or two after work every weekday, for example, you'll quickly notice that the same people are there with you. Interact with them enough, and these people form the outer ring of your social life.

Then you need to have explicitly social gatherings. For example, I've got a biweekly game night that an old friend and I alternate hosting. We invite our friends and the occasional acquaintance from our outer rings, and some start coming regularly. Typically somewhere between 4 and 10 people show up, BYOB, and we play some board games into the odd hours. Often random subsets of the group will grab dinner together before hand.

Then you have your close friend maintenance regimen. People you're not seeing regularly anymore in the context you used to (whether it was school, and old workplace, etc), but that you want to maintain a relationship with. If you spend at least 1 weekend night a week grabbing dinner and some drinks with one of these people, you'll be able to maintain your relationships with up to a half dozen or so people (in addition to the ones that are in your more active group).

Mix in some larger gatherings/parties a couple times a year, and you've set yourself up a healthy social life.