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.)
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. Finally, apply Trinisphere's effect if the mana component of the spell's cost is less than three mana. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.
07-08-2020
wotc
If Trinisphere leaves the battlefield or becomes tapped or untapped as a cost to cast a spell, this cost is paid after you've locked in the total cost.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.
to cast costs 
AKQA2024-04-08
