Replacing galvanized
Pay now or pay later. Galvanized pipes inevitably fail over time because they crud up from the inside which constricts flow. This reduces the thickness of the wall, so if the flow doesn't get you, the failure of the wall will.

Having said that, 3" is huge, so it would probably take a while before you noticed flow constriction. You'd still have a problem with wall failure.

It sounds like you're putting up a wall finish now, so fixing now would be cheaper, as you wouldn't have the future process of [demo/ re-pipe/ re-finish].

If you knew how old your existing pipe was, you might have a shot at predicting its lifespan.

See also: Galvanized pipe life expectancy