Trinisphere

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.

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.