The Baldur’s Gate story has already been completed. I doubt the part III will continue with the prior story as it takes place over 100 years after the first two games. I believe it’s more of a marketing ploy than anything else. Like how most of the Final Fantasy games are not related to each other. It’s just another story within the city of Baldur’s Gate.
If you are going to play through though. I recommend BG1 then BG2 then the xpac Throne of Baal. It’s BG2 when the series starts to shine.