In the past, your opponent having a Trinisphere meant that, no matter what, you were paying 3 mana or more for your spell with delve. However, this is no longer the case.
Since delve no longer reduces the total cost of the spell, this means that Trinisphere will be checking to see if the spell is below a total cost of 3 before you use the delve ability to exile cards instead of paying for the generic mana.
So let’s say you want to cast Dig Through Time, exiling 6 cards and paying UU. Trinisphere will check the total cost before you start paying for the spell to determine if it needs to increase that total cost to include at least 3 mana being paid. Since the total cost for Dig Through Time is still UU6, Trinisphere is happy that it’s more than 3, and doesn’t interfere. Then when you get to the actual “pay costs” part of the spell casting process, you spend UU and exile 6 cards from your graveyard. Trinisphere might become confused about how you managed to cast a spell by only paying 2 actual mana, but it’s too late to go back and interfere now!