To catch up with some lore and the past, is it worth it to start BG1/BG2 while BG3 progress with patches and content ?
No, and yes.
There's no immediately obvious connection to them and Baldur's Gate III beyond the overall theme of 'how much are you willing to sacrifice for power'. There are references to 'gorians ward' and minature giant space hamsters in some books, but nothing else is mentioned and the description of the protagonist from BG1 is very deliberately nothing beyond "They existed".
There's going to be a bit of 'culture shock' with the rules however (2nd edition that they run off has a 'low is good, high is bad' system, so your armour class drops when you put on plate mail!). I'm also not blind to how western rpgs have moved on; inter party dialogue is basically non existent, and while the sprites and maps have a charm, they and the interface definitely show their age.
I think it's worth it because I enjoy the characters and the story is genuinely interesting, the characters are memorable, and the gameplay itself still holds up. It doesn't seem necessary however, and unlike Bethesda with Fallout 3 vs fallout 1 and 2, Baldur's Gate III isn't going for a fundemental shift of game types. If you're interested in games from a design point of view, I would absolutely reccomend you try it, for a game on it's own, a classic, I'd also reccomend it, but there's no obvious connection to Baldur's Gate III, so I'd skip it if you're only thinking of it for that.
If you do pull the trigger, I reccomend you try and avoid Beamdog's version-while some of the things they did were great, my understanding is they also added entirely new bugs.
