A1: Yes, you have to pay 3 for Trinisphere.
A2: Yes, you have to pay 1 for Sphere of Resistance.
B1: Yes, you have to pay 3 for Trinisphere.
B2: Yes, you have to pay 1 for Sphere of Resistance.
B3: Yes, you have to pay 1 for Thalia or Thorn of Amethyst.
C1: You must spend at least 3 mana no matter what. You cannot pay 2 life because you are casting the spell for a cost other than it's casting cost and so can't also pay it;s casting cost in addition to that alternative cost. Incidentally if you were just casting Gitaxian Probe from your hand you may also pay 2 life if you want but you cannot pay less than 3 mana even in that case. If you wanted to pay 2 life you will still pay 3 mana on top of the two life when casting it from your hand without using As Foretold.
C2: You must spend at least 1 mana. You may spend only one mana and two life or you may spend two mana, one of which is blue. Your choice.
D: No, you cannot replace the flashback cost for a card using As Foretold. You may only replace the casting cost of a card using As Foretold. Flashing back a card is an ability that allows you to cast an enabled instant or sorcery from your graveyard whereas casting a spell is a special action. If you were targeting the Force of Will using a Jace, Vryn's Prodigy activation you would be able to cast it for free because that ability allows you to cast the card using any available means rather than giving the card flashback.