The invention relates to the technical field of gas and liquid separation, in particular to a three-phase separator system used in a gas well liquid discharging process and a using method. Different from an existing control method, 1, a liquid level is not controlled to run at a certain height in a separator, but the liquid level is controlled to run between a liquid outlet and a gas outlet; 2, the two gas and liquid outlets adopt pneumatic pressure adjusting valves of the same model and the same parameter for control, so that the two outlets are equal in adjusting pressure, and it is guaranteed that the separator is steady in work pressure; 3, the gas pressure adjusting values of the two gas and liquid outlets are controlled through one pressure adjusting valve, so that the two pneumaticadjusting valves are equal in control gas pressure, and therefore the pressure is equal, that is, the pressure in a separator tank is smooth and steady, and good separating is guaranteed; and 4, whenthe gas outlet contains liquid drops, it is only needed to adjust the control gas pressure of the pressure adjusting valve high, and then gas and liquid separating can be achieved; and if the liquid outflow and gas outflow amounts are both very small, the control gas pressure of the pressure adjusting valve is adjusted small to lower return pressure on a well opening, and then normal liquid discharging of a gas well can be achieved.