I'm trying to think, as it currently stands, how they would even add Counterspell into BG3 right now. And perhaps this, and other L3 spell mechanics, is why the level cap is at L4 to postpone having to deal with Larian having to think about things like this.
I can't even wrap my mind around how they could implement Counterspell "Larian style" and make it seem impactful. I honestly think they would make it where like "creatures in x range of you cannot cast spells for 1 turn" or something. LOLOLOLOL.
The spells and mechanics only get more complicated as levels increase, as multi-classing comes into play.
Well, since I don't know the technical details of the engine, I can only judge based on what is in the game currently. If Larian were to implement reactions using what's currently in the game, then the wizard toggles on Shield and Counterspell in the Reactions tab. A goblin shoots a fire arrow at your wizard. The game forces your wizard to cast Shield. The fire arrow misses, hits the ground at his feet, sets it on fire, and does 2D4 guaranteed damage to the wizard that he can't roll Dexterity save against, and then rolls a Constitution save to keep maintaining his Concentration spell that he'd previously cast. In the next round, a goblin booyagh casts Sacred Flame at the wizard, forcing him to cast Counterspell, burning a level 3 spell slot to nullify the enemy's cantrip.
LOLOLOLOL. Or it's just a targetable buff you can place on yourself or others that negates the effect of the next spell cast on target. No reaction mechanic implemented.