MAGIC: THE GATHERING
As long as Trinisphere is untapped, each spell that would cost less than three mana to cast costs three mana to cast. (Additional mana in the cost may be paid with any color of mana or colorless mana. For example, a spell that would cost to cast costs to cast instead.)
Trinisphere is a very strong taxing artifact especially in a format where Mishra's Workshop is playable which is why Vintage is restricted to one copy per deck. Don't let Vintage fool you though. Trinisphere is still very viable in Legacy and some Modern decks as a way to kick the price up on some of those pesky spells.