Trinisphere and Sphere of Resistance/Thorn of Amethyst

[Bera]

How much mana does it cost to play a mox pearl when there is a sphere of resistance and a trinisphere in play? I'm pretty sure it's three mana but a friend says it's four.

[Shock Wave]

3.

[Yare]

Rule of thumb: Always apply Trinisphere last. So, after doing all of your adding and subtracting, Trinisphere will look at the resulting mana cost and go "Do you cost 3 or more? If you cost 3 or more, I don't do anything. If you cost less than 3, you now cost 3."

[Rico Suave]

You always apply cost increasers first, cost reducers second, and Trinisphere last.