Does a creature enchanted with Pacifism still deal combat damage if it's already attacking when Pacifism resolves?

It depends on timing. Pacifism states the enchanted creature "can't attack or block." This is a restriction on declaring the creature as an attacker or blocker. It does not remove a creature from combat once it is already legally attacking or blocking.

According to CR 509.1, attackers are declared during the declare attackers step. Once a creature is declared as an attacker, it becomes an attacking creature (CR 508.2) and remains so until combat ends or it leaves the battlefield. Pacifism resolving after the creature has already been declared as an attacker does not retroactively remove it from combat.

CR 506.4 clarifies that a creature that is already attacking or blocking isn't affected by effects that say it "can't attack" or "can't block" — those restrictions only apply at the moment of declaration. Therefore, if Pacifism resolves after the declare attackers step, the enchanted creature will still deal and be dealt combat damage normally during the combat damage step (CR 510.1).

Concrete example: Your 3/3 creature attacks your opponent. In response to Pacifism being cast (before it resolves), you do nothing. Pacifism resolves during the declare blockers step. Your creature is still an attacking creature — it will deal 3 combat damage to your opponent (or its blocker) as normal that turn. Next turn, it cannot be declared as an attacker.

In summary: Pacifism stopping an already-attacking creature from dealing damage is a common misconception. The enchantment prevents future attacks and blocks, but has no effect on a combat the creature is already participating in.