This paper presents a new low-cost and efficient approach for the real-time three-dimensional (3D) binaural audio capture and reproduction via headphones using a miniature microphone array. The microphone array is configured in B-format to minimize space requirement using an omnidirectional microphone and three bidirectional microphones. The signals captured by the microphone array are applied by a set of optimal time-invariant gain vectors, which converts a B-format Ambisonic sinal into binaural signal for headphone reproduction. The optimal time-invariant gain vectors that are computed offline integrate the two stages of beamforming and head related transfer function (HRTF) filtering. As an alternative to the virtual speaker method, the proposed beamforming approach is independent of the number of virtual audio sources and flexible for working on different sets of HRTFs. A real-time system has been implemented based on the proposed method. Psychophysical hearing tests show good localization accuracy.