Overcrowded Prison

When Overcrowded Prison enters the battlefield,
name a creature type. Whenever a player cast that
creature type, exile it. Then if 3 or more creatures
were exiled this way, return all creatures exiled by Overcrowded Prison to their owner's hand and
destroy Overcrowded Prison.
{t}:Add one mana to your mana pool of any color
for each creature exiled by Overcrowded Prison.