In microwave-assisted magnetic recording (MAMR), it is still a challenge to obtain a large microwave-assist field (MA field). It is also not easy to enhance the static head field gain (DC gain) brought by the effect of recording flux control. In this paper, we propose a tri-layer spin-torque-oscillator (STO) made of negative spin-polarization material, and we compare the MA field and the DC gain with a conventional dual-layer STO. With this new design, a field generation layer (FGL) can easily obtain stronger spin-transfer torque to overcome the magneto-static interaction with head poles. It works well to increase the DC gain without losing the MA field at lower injection currents. A remaining problem is a significant reduction in the MA field when the injection current becomes too large.