gibber

"I have three mana, you played a one drop and a 2 drop? Guess I win the game then."

Are you saying it's OP? You always speak in riddles!

Ok; so. Unconditional exile forever. White isn't allowed to do that.

Time-out exile for a while? Sure. Exile because you do something 'evil' (like attacking, say) sure. Pre-emptive "I don't like the look of that?" No.

Add to that the "If I keep casting this, you never cast another spell ever again" effect? White can do that, permission is a white thing - but that's a 6 or 7 mana effect, and without a get-out clause of some kind very uninteractive and ufun (but hey, at 'end game' mana, not so much of a problem, since then it's a win, not a lockdown). It's certainly not a 3.

Would flicker fix this? Maybe if I add a clause to exile this spell

No, there's no reason to exile this spell. White doesn't get spell recursion anyway. There just needs to be some restriction on what it can target, maybe "power 4 or greater" or "tapped"

I have no idea what Vitenka was talking about in their first comment

The first comment is the "I dream of" scenario where this is most broken. You ghave two in hand, three mana in play; and a black player has a 1-drop and a 2-drop in play.

They're looking at missing their next two turns. Might as well walk away. (Though it's probably actually better to cast this on turn 5 or so when you can hit a bigger creature and cost them a more important turn.)

Oh, gotcha.

Okay so Exile target creature that attacked this turn? I don't know, it's the most flavorful option I can think of right now. But then it would be better as an instant.

Exile target creature with CMC 4 or less? Power 4 or less? Toughness 4 or less? Idk, I like that this scales in strength with the game..

Maybe exile target creature that entered the battlefield this turn? that's interesting enough...