We have designed, fabricated, and characterized a fiber-optic three-dimensional vector accelerometer for vibration measurement. The accelerometer is based on fiber Bragg gratings (FBGs) inscribed with three cores of seven-core fiber by femtosecond laser with a phase mask. Through monitoring the wavelength shifts of two of the seven cores, we obtain the vibration azimuthal angle and the acceleration in the XY plane by angle reconstruction and algorithmic calculation. Similarly, by monitoring the wavelength shift of the core fixed along the Z-axis, we obtain the vibration acceleration in the Z-axis direction. The full 3D vector information about the vibration signal is obtained by vector synthesis. The accelerometer has achieved a working frequency bandwidth from 10 to 220 Hz with the maximum sensitivity of 355 pm/g, and the best azimuthal angle accuracy of 0.269 ^° in the linear range of 0 ^° -100 ^° and 170 ^° -280 ^° . Moreover, it is possible to increase the overall bandwidth, sensitivity and accuracy of the azimuth of sensors in the XY plane by adjusting the length of the optical fiber and adjusting mechanical parameters to increase the bandwidth and sensitivity of the sensor in Z-axis. Such improvements will further enhance application to small-scale seismic surveys.