
Instant — Trap
If an opponent cast three or more spells this turn, you may pay rather than pay this spell's mana cost.
Exile any number of target spells.

Cheap instant counter great against storm and multiple spells on the stack.

Free counterspell answer to combo turns without mana.

Free counter for noncreature spells protecting against combo.

One-mana counter for instants/sorceries common in storm decks.

Free counterspell at instant speed to stop a combo turn.

Uncounterable hard counter for noncreature spells.

Cheap interaction targeting triggered/activated abilities during combos.

Reliable hard counter for any single spell.
Mindbreak Trap's alternative cost condition checks whether an opponent cast three or more spells this turn, not whether those spells have resolved.
For Mindbreak Trap's alternative cost to apply, a single opponent must cast three or more spells. Three opponents each casting a single spell won't work, for example.
If a spell is exiled, it's removed from the stack and thus will not resolve. The spell isn't countered; it just no longer exists. This works on spells that can't be countered.