This paper presents the design of a monolithic 3- axis thermal accelerometer using a standard CMOS process. Based on free convection in a sealed cavity, the sensing principle is well known and many examples of single and dual-axis MEMS accelerometers can be found in the literature. More recently, Finite Element Modeling has demonstrated the feasibility of a fully integrated 3-axis sensor manufactured with a simple post-process to partially etch silicon bulk of a CMOS die. A prototype was designed and fabricated; this paper presents the sensing cell, its associated conditioning electronics and preliminary characterization.