A new detector bridge design used in a CMOS MEMS 3-axis thermal accelerometer is proposed in this work. The new design consists of four detector bridges which are presented and optimized. At first, we used FEM simulations to find optimal detectors locations for maximum in-plane and out-of-plane sensitivities. Then, the design of detector bridges is optimized through the investigation of the effect of different designs on the sensitivity. The one using the minimum oxide volume offers the highest sensitivity. This latter is compared with values obtained from a structure using conventional square-shaped heater and detector bridges. Higher sensitivities are demonstrated along the three axes for different heating temperatures. The new structure offers maximum in-plane and out-of-plane sensitivities of 254 mK/g and 20 mK/g, respectively. The resistances of the detectors are designed for the AMS 0.35μm CMOS fabrication process and are implemented in an electronic conditioning chain.