Oracle Text

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 1B to cast costs 2B to cast instead.)

Card Rulings

12/1/2004 Trinisphere’s ability affects the total cost of the spell. It is applied *after* any other cost increasers or cost reducers are applied: First apply any cost increases. Next apply any cost reducers. Finally look at the amount of mana you have to pay. If it’s less than three mana, you’ll pay three mana.
12/1/2004 Even with a cost reducer on the battlefield, spells can’t cost less than three mana to cast.
12/1/2004 If a spell costs at least three mana due to additional costs, such as kicker costs, that’s fine.
12/1/2004 You still need to pay any additional nonmana costs the spell has, such as sacrificing a creature or discarding cards.
12/1/2004 Casting a creature with morph face down already costs three mana, even though the converted mana cost of the face-down spell is zero, so Trinisphere normally doesn’t modify the total cost of a face-down creature spell. However, if Dream Chisel is reducing that cost while Trinisphere is on the battlefield, you’ll still have to pay three mana for the spell.