A three-axis inertial sensor and a process for its fabrication using an silicon-on-oxide (SOI) wafer as a starting material. The SOI wafer has a first conductive layer separated from a second conductive layer by an insulative buried oxide (BOX) layer. The SOI wafer is fabricated to partially define in its first conductive layer at least portions of proof masses for z, x, and y-axis sensing devices of the sensor. After a conductive deposited layer is deposited and patterned to form a suspension spring for the proof mass of the z-axis sensing device, the SOI wafer is bonded to a substrate that preferably carries interface circuitry for the z, x, and y-axis devices, with the SOI wafer being oriented so that its first conductive layer faces the substrate. Portions of the BOX layer are then etched to fully release the proof masses.