A new optimized heater design for a 3-axis thermal accelerometer is proposed. The new heater geometry is optimized to improve in-plane and out-of-plane efficiencies in terms of sensitivity per unit of power, while insuring CMOS-MEMS process compatibility. This geometry assembles a hollow square-shaped frame with four wings placed on each of its angles. Four heating resistances are implemented in the wings' extensions in order to concentrate the heat generation close to the detector's region. A complete finite element model is developed for both sensitivity and dissipated power investigation as well as efficiency evaluation. The sensor efficiency is enhanced through the optimization of the heater structure. Compared to the conventional single central heater design, better sensitivities are demonstrated along the three axes with lower heating temperature. The same sensor performance is found at lower power consumption, while higher sensitivity is achieved at the same power consumption. The sensor efficiencies are thus enhanced. Moreover, the new design permits the use of much higher input power. A power up to 17 mW can be used according to the targeted application, producing in-plane and out-of-plane sensitivities up to 255 mK/g and 20 mK/g.