You can not twin fireball, but you can quicken it for the same cost, and then cast it with your bonus action again.
And then you can use Potions of Speed and/or Haste to cast a 3rd fireball!
Ok, remind me how that works? The rules say, iirc, you can't cast two leveled spells, only a leveled + cantrip, ever. So the Quicken rule does't change that - it simply talks about moving casting to a bonus action instead. That is how Solasta implements it - I played a sorcerer there - and that seems to be the description that is consistent with rules? That's how I've always read it. It is only 2 sorcery points to quicken - which is a tiny cost to pay, so not sure how any DM would interpret this to be Quicken in the old 3.5/PF sense (where it was spell level +4 , so hugely costly). Well, let me rephrase that any DM but larian ;-)
No, you have it 100% (well, actually ~90%) correct. Especially that last line.
Larian made changes to
- general spellcasting rules (normal BA spells can be cast on the same turn as action-cost leveled spells)
- Quicken rules (quickened BA spell can be cast on the same turn as an action-cost leveled spell)
- Haste (instead of allowing only a single attack action/Dash/Disengage/Hide/Use Object, BG3 allows you to cast a spell with your hasted action)
Technically, per 5e RAW, you *can* cast two leveled spells per turn as long as neither of them is a bonus action. Counterspell and Fireball? Fine. Action-surged Fireball+Fireball? Fine.
