If the target creature's power is increased to 3 or greater after Deserter's Disciple's ability is activated but before it resolves, the ability doesn't resolve. However, if instead the creature's power is increased to 3 or greater after the ability resolves, it still can't be blocked that turn.

Once a creature with power 2 or less has been blocked, Deserter's Disciple's ability won't cause it to become unblocked.