I don't know if there will be other towns or villages in BG3 but I'd like it if so. In the first game there were Beregost and Nashkel in addition to Baldur's Gate itself of course, in addition to smaller settlements, wayside inns, etc, and in BG2 there was ... was it called Tradesmeet? ... and I think a small village in the Umar Hills as well as Athkatla, and I'm sure other bits I've forgotten. So while I think it's very much in the spirit of its predecessors to have a fair amount of tramping around in the wilderness and abandoned ruins, and in the spirit of BG1 specifically to have a fairly long build-up to getting to the big city, I hope it will also follow them in not just having one settlement.
That said, the amount of work that needs to go into creating a town in BG3 will be significantly more than went into those earlier games, so I wouldn't blame Larian if they made Baldur's Gate itself the only major centre of civilization, with hopefully the odd wayside inn (that isn't on fire!) and trading post along with smaller settlements like the druid grove, goblin camp and Myconid Village. And I suspect that even with Baldur's Gate itself, it may not be feasible to explore the whole city, though I do hope that it will have different zones/neighbourhoods that we can go to, as with Baldur's Gate and Athkatla in the earlier games.
I don't think we know anything for certain yet, though, except that we will get to see (at least part of) Baldur's Gate itself.
