Just the Pitt and the commonwealth tho
Ronto, the Erie Stretch, Broken Banks/Crater Banks, Great Lanta, Montana Chapter of the Brotherhood, and yes as you say, the Commonwealth (I wouldn't really count the Pitt since we actually get to see it in the game).
Fallout 3 generally did a very good job of throwing out these little hints and suggestions of a wider world by giving interesting names but taking them for granted. It's sort of a cheap strategy, but it's very effective.
Really Fallout 3 makes far more references to locations and regions outside the main world than Fallout 1 and 2 that have practically none at all. Fallout 1 I think has nothing in terms of post-War locations. Fallout 2 has... Lake LeBarge? But provides no intrigue, it's just an IRL location someone has to be from. Fallout 2 does also have it's mentioned-only discussions of the things going on in the territory of Fallout 1, though that's a little different given that's an area we've already seen, just now in the future and just offscreen.
Fallout 3 ends up undermining it's wider-world-building in it's local-world-building, however: Because the economics and dynamics of the Capital Wasteland are so poorly thought through and nonsensical, it doesn't feel like a cohesive part of the world in itself, and it can't be conceived of as having any relationship with a wider world. Names are just names, cast amid a void. Fallout 1 and 2, on the other hand, are fairly well constructed locally, so it is believable that there's a wider world out there, even if we hardly if ever hear it discussed.
Fallout New Vegas has both a very well constructed local worldbuilding, well thought out relationships with the areas just off screen (core NCR and Legion), and finally scatters in an absolute ton of references to even more far-flung locales and goings-ons. All of this builds an illusion of a wider world more effectively than any other title in the IP.