Casting Salvage Titan by paying its alternative cost doesn't change when you can cast it. You can cast it only at the normal time you could cast a creature spell.

You may activate its second ability only if Salvage Titan is in your graveyard. To pay this ability's cost, you may exile any three artifact cards from your graveyard—including Salvage Titan itself. If you exile it to pay the cost, however, it won't be returned to your hand when the ability resolves.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as Salvage Titan's alternative cost), add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.