I was using Flamebreak during the MTG pc game, and I had a 5/4 without flying. I used it, and the 5/4 died! Why? It says it only deals 3 dmg to non-flying creatures.
We can't tell why your creature died from the limited information you gave because Flamebreak alone would not have killed your creature.
- Maybe your creature's toughness was being buffed by the static ability of some other creature that died from Flamebreak.
- Maybe your creature already had damage marked on it.
- Maybe your creature received damage from something soon after Flamebreak.
- Maybe something debuffed your creature's toughness.
- Maybe something doubled the damage.
- Maybe ...