Spin-transfer torque-based magnetic random access memory is becoming more and more attractive in industry due to its non-volatility, fast switching speed and infinite endurance. However, it suffers energy and speed bottlenecks, so the magnetic tunnel junction urgently needs a new write scheme. Compared with the spin-transfer torque, emerging spin-orbit torque will replace spin-transfer torque as a new write scheme of magnetic storage technology for its faster writing speed and avoiding the barrier breakdown. A three-terminal magnetic tunnel junction consists of magnetic tunnel junction/heavy metal structure offers a promising perspective from a technological point of view in the design of new generation of magnetic random access memory, for it is possible to control the magnetization dynamics through two current densities of spin-transfer torque and the spin-orbit torque. In this paper, the stability of magnetic states in the three-terminal magnetic tunnel junction is studied theoretically. Through linearizing the Landau-Lifshitz-Gilbert equation including the spin-transfer torque and the spin-orbit torque defined in the spherical coordinates, the new equilibrium directions and linear differential equations are obtained. Performing linear stability analysis of the new equilibrium directions, the phase diagrams defined by the direction of pinned-layer magnetization vector and the current density of spin-orbit torque are obtained. Several magnetic states are distinguished in the phase diagram, such as in-plane precessional and stable states, out-of-plane precessional and stable states. When the pinned-layer magnetization vector rotates out of the film plane, through adjusting the direction of pinned-layer magnetization vector, the switching from stable state to precessional one can be realized. Orientating the pinned-layer magnetization vector in the film plane, neither the out-of-plane precession nor stable states emerges for the current density of spin-orbit torque and spin-transfer torque are relatively small. The instability current takes a minimum value with the pinned-layer magnetization vector nearly parallel or antiparallel to the easy axis of free layer and increases with the direction of pinned-layer magnetization vector deviating from these two locations. The magnetization reversal can be realized through adjusting the current density of spin-transfer torque, and the reversal time can decrease greatly under the assisting of spin-orbit torque. By showing the dependence of magnetization vector on the time of different magnetic states, the validity of phase diagram is confirmed. The selecting of the different directions of the pinned-layer magnetization vector provides an alternative way to control the current-driven magnetization dynamics. This will provide useful guide for the application of three-terminal magnetic tunnel junction.