We present a fast mid-air gesture keyboard for head-mounted optical see-through augmented reality (OST AR) that supports users in articulating word patterns by merely moving their own physical index finger in relation to a virtual keyboard plane without a need to indirectly control a visual 2D cursor on a keyboard plane. To realize this, we introduce a novel decoding method that directly translates users' three-dimensional fingertip gestural trajectories into their intended text. We evaluate the efficacy of the system in three studies that investigate various design aspects, such as immediate efficacy, accelerated learning, and whether it is possible to maintain performance without providing visual feedback. We find that the new 3D trajectory decoding design results in significant improvements in entry rates while maintaining low error rates. In addition, we demonstrate that users can maintain their performance even without fingertip and gesture trace visualization.