Trinisphere and Sphere of Resistance

[Motrax]

Are Trinisphere and Sphere of Resistance cumulative? For example, if one of each sphere is in play, would an Ancestral Recall cost 3 or 4 mana to play?

For reference:

Trinisphere -  
Artifact (R)
As long as Trinisphere is untapped, each spell that would cost less than three mana to play costs three mana to play. (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 play costs 2B to play instead.)

Sphere of Resistance
Color= Artifact  Type= Artifact  Cost= 2 EX(R)  
Text (EX): Spells cost more to play. [Oracle 1999/05/01]
It affects all players, including you. [DeLaney 1998/06/10]

[jpmeyer]

Three.  It looks at how much mana you would be spending, which is 1U, and says "gotta be 2U."  Trinisphere is the last thing you apply.