I really need to get around to playing D:OS1 & 2, are they much the same as BG3?
They're similar in that they're isometric turn based RPGs, but gameplay mechanics are very different.
With Divinity you don't have any clearly defined classes, instead you have skill points which allow you to unlock higher level abilities of certain skills (e.g. if you want your melee fighter to be able to cast a basic healing spell then you could put 1 point into water magic - or simply equip them with an item which gives that 1 point) whereas with BG3 you're very much restricted to your class abilities (obviously you can multi-class).
Your spells don't have limited uses which need to be regenerated by resting, instead you have the "standard" cooldown mechanic which many games use, so while you can't constantly spam spells, you also don't need to return to camp to rest after every fight, or risk entering a difficult fight with half your abilities missing.
In combat, you don't have to juggle movement/actions/bonus actions/etc. and which abilities use which, you have action points (AP), which are shared between everything, and gives you a bit more flexibility in what you can do each turn. So e.g. if you had 6AP, you could move a certain distance for 3AP and then attack for 3AP, or do 2x 3AP attacks, or cast a 2AP spell, move for 1AP and then attack for 3AP, etc.
I really enjoyed D:OS1, but I'm struggling to get into 2; there are some "unique" armour mechanics, which mean mixed parties of spellcasters/physical damage are a bit nerfed vs focusing purely on one or the other - a fact I only learned recently when trying to find out why I'm really struggling with fights which should be tough-but-winnable at my level - I need to re-spec. a couple of characters or swap them out I think.
BG3 seems to have a lot more meaningful choices and character development, whereas D:OS1&2 are more linear/fixed storyline
I did also really enjoy BG3, but I played that exclusively with a few mates, which obviously makes it a lot more fun (Divinity I believe is limited to 2 players, whereas BG3 allows 4*)
*or up to 8 with some simple modding!