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.)
"This exhibition of Nehra's Threshold Principle must be expropriated and examined for its potential applications in enforcement."n—Confiscation directive
to cast costs 
Daniel Ljunggren