1. An ultrasonic imaging system for generating a three-dimensional Doppler image, comprising: a scanhead comprising: at least three receive apertures arranged in a common plane; and a transmit aperture; and an imaging unit comprising: a beamformer coupled to the receive apertures, the beamformer being structured to combine signals from at least two of the receive apertures to image an area adjacent the scanhead; a Doppler processor for each of the receive apertures in the scanhead, the Doppler processor being coupled to receive a signal from the beamformer and being structured to generate a magnitude signal indicative of the Doppler motion magnitude of moving ultrasound reflectors in the imaged area and a direction signal indicative of the direction of the moving ultrasound reflectors in the imaged area; a velocity estimator coupled to receive the magnitude and direction signals from each of the Doppler processors, the velocity estimator being structured to generate a magnitude signal indicative of the magnitude of a three dimensional motion vector corresponding to the magnitude signals from the Doppler processors and a motion angle signal indicative of the direction of the three dimensional motion vector corresponding to the direction signals from the Doppler processors; a display processor coupled to receive the three-dimensional magnitude signal and the three-dimensional motion angle signal from the velocity estimator, the display processor converting the three-dimensional magnitude and motion angle signals to a display signals having a predetermined display format; and a display coupled to receive the three-dimensional magnitude and motion angle signals from the display processor, the display providing a the three-dimensional image of motion in a volume adjacent the scanhead.