BG3 will feel like an entirely complete game on its own. It is in the same world and has some recurring characters, but the game is designed for newcomers so they don’t assume that you know the characters and history.
For those who have played BG1/2, it starts off feeling more like a spiritual successor imo. But eventually you see how it all actually fits, and it fits very nicely. But that’s more of a bonus for return visitors to the Forgotten Realms because the game stands just fine on its own.