I haven’t played any Baldur’s Gate games before but I’ve heard so much about this game that I’m going to buy it.
However, before I start, I always wonder about this: some games allow you to unlock any weapon at some point in the game, and if you miss one in some quest you can always go back. If you accidentally sell one you can buy it back or forge a new one again, or have it respawn. If you want some other class you can switch later.
Other games are not like that, and if you screw up or aren’t aware of [full in the blank] then you can’t unlock something.
What’s the story with BG3? Do I need to be careful and plan before going on missions?
Just to further illustrate your point to OP, I’m deep into Act 3, nearing the end of the game, and I’ve found a single +3 AC shield in the entire game. Even the super late game bosses drop a shield with the standard +2 AC and some unique and powerful effect, rather than a “better shield”.
Yes, there are unique weapons you can miss out on. None of them are so unique or powerful that your experience will suffer greatly because you missed it. In one case, a side quest allows you to craft 2 of like 7 potential pieces of equipment and doesn’t give you enough material to craft any more. That’s the nature of the game: you have to make decisions and work with what you find. On the plus, this makes sharing the experience with friends and playing the game multiple times a substantially better experience.