Prevent the next 3 damage that would be dealt to target creature this turn. For each 1 damage prevented this way, put a +1/+1 counter on that creature.
The +1/+1 counters are put onto the creature at the same time the damage is prevented. If a 1/1 creature would be dealt 6 damage, 3 damage is prevented and three +1/+1 counters are put on the creature. The creature ends up as a 4/4 creature with 3 damage marked on it.