Players can't gain life this turn. Damage can't be prevented this turn. Call In a Professional deals 3 damage to any target. (Shield counters don't prevent this damage as they're removed.)
If Call In a Professional deals damage to a creature with a shield counter on it, that damage will not be prevented, but a shield counter will still be removed. If that damage is enough to be lethal damage, the creature will be destroyed, even if it still has more shield counters on it.