Luffy’s use of Gear 3 is unique because the audience does not actually see the transformation the very first time it happens.
The form is fully shown for the first time during Luffy's climactic battle against Rob Lucci . In this fight, Luffy explains the "Bone Balloon" mechanic and uses the "Gomu Gomu no Gigant Pistol" to nearly knock Lucci unconscious. How Gear 3 Works: Bone Balloon when does luffy first use gear 3