Although conventional Doppler ultrasound is widely used for quantifying blood flow, it is restricted by its low sensitivity to detect slow flow. The incorporation of ultrafast ultrasound and spatial-temporal clutter filters can not only extensively boost the Doppler sensitivity to low-velocity slow flow but also facilitate the development of advanced 3D Doppler techniques. In this work, we propose a novel 3D Doppler method which extends 2D imaging to 3D through the continuous mechanical translation of a linear transducer. The viability of this method is assessed by simulations with the aids of a theoretical model. The combination of simulations and the theoretical model provides unique insights into the inherent mechanisms involved in the performance of this 3D Doppler method and the roles of factors, such as tissue vibration characteristics, blood flow velocity, elevational Point-Spread-Function profile, probe translating speed, and signal energy ratios.