Runaway Steam-Kin

Whenever you cast a red spell, if this creature has fewer than three +1/+1 counters on it, put a +1/+1 counter on this creature.
Remove three +1/+1 counters from this creature: Add {R}{R}{R}.

Guilds of Ravnica Promos

Guilds of Ravnica

Whenever you cast a red spell, if Runaway Steam-Kin has fewer than three +1/+1 counters on it, put a +1/+1 counter on Runaway Steam-Kin.
Remove three +1/+1 counters from Runaway Steam-Kin: Add {R}{R}{R}.

If Runaway Steam-Kin has three or more +1/+1 counters on it after you’re done paying for a red spell, its ability doesn’t trigger at all. If it has three or more +1/+1 counters on it as its triggered ability resolves, it doesn’t get a +1/+1 counter.

Runaway Steam-Kin has to be on the battlefield for its ability to trigger. Casting it won’t trigger its own ability.

Runaway Steam-Kin’s triggered ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.