Rule 608.2c. starts with "The controller of the spell or ability follows its instructions in the order written.", and Skullcrack's effects read
Players can't gain life this turn. Damage can't be prevented this turn. Skullcrack deals 3 damage to target player.
Does the second sentence causes the damage from the third one to become impossible to prevent?
As you quote in rule 608.2c, you resolve a spell or ability, effects in different sentences are resolved consecutively. When Skullcrack resolves, first it creates a continuous effect that stops players from gaining life for the rest of the turn. Then it creates a continuous effect that makes damage unpreventable for the rest of the turn. Finally, it deals the damage, which cannot be prevented because the effect stopping prevention already exists.
You are correct that the damage cannot be prevented. Anything that says it would prevent the damage no longer works as soon as the spell resolves.
