This paper reports efficiency modeling using 3D FEM simulation of a convective accelerometer obtained by FSBM of a die fabricated using standard CMOS technology. In such sensors, best sensitivity is obtained by placing temperature detectors where air temperature is the most sensitive to acceleration. This will obviously depends on 3D effects. In a previous work, a behavioral model of the sensor including only 2D effects was developed. This work investigates 3D effects which give the opportunity to better predict not only sensor sensitivity but also power dissipation. Experimental sensitivity values and 3D FEM ones are verified for two different sensors and two different heater temperatures. For a prototype having a heater-cavity border distance of 340μm and a heater length of 230μm, maximum sensitivity point is obtained for detectors localized at a distance of 125μm from heater center. Using this 3D geometry in FEM simulations, we show that electrical power decreases more rapidly than sensitivity when heater length is reduced. Moreover, when detectors are shortened, the sensitivity will be quite higher with an optimal value obtained for a detector implemented on one third of the side bridge.