A question about Trinisphere

[tysxych]

There are two Trinispheres in play. I play a spell with 1 coverted mana cost. How many extra mana shall I pay?
Thanks.

[Nydaeli]

You pay three mana.  Multiple trinispheres don't have any additional effect.

[Apollyon]

You have two of the same ability in play, both of which are setting the mana cost of a card. So you apply them in the "Trinisphere layer" of determining mana cost of a card (you first apply cost increases, then you apply cost reductions, then you apply Trinisphere).

As an example, you have a Metallic Sliver (1/1 artifact creature for 1) and 2 Trinispheres out. You announce Metallic Sliver, put it on the stack, skip over all of the optional stuff and get to mana cost. Metallic Sliver costs 1, there are no cost increases, there are no cost reductions, and then you apply Trinisphere, and the result is that Metallic Sliver costs 3 to play, regardless of how many Trinspheres there are in play.