@prn Delve used to function as cost reduction but it was changed around Khans. The current rules are below.
From Dig through Time:
Delve (Each card you exile from your graveyard while casting this spell pays for 1.)
From Gitaxian Probe
(Phyrexian Blue can be paid with either Blue or 2 life.)
Trinisphere cares about how much mana is paid for a spell, and the way Delve and Convoke are worded, removing a card or tapping a creature counts as paying mana. Phyrexian mana contains "or" so you are given the option of paying mana, which obviously satisfies 3Ball, or paying life, which does not. Alternate costs are paid instead of the spell's mana cost. You chose at the beginning whether or not you are going to pay 3UU of Remove a Blue Card and pay 1 life for your Force of Will. Delve, Convoke, and Phyrexian mana are not alternate costs, which is why with Snapcaster Mage , which grants Flashback equal to the spell's mana cost, you can use Delve, Convoke, and Phyrexian mana to pay for the Flashback.