The 3 MEMS structures are arranged orthogonally, on the X, Y and Z axis. Deflection on each tuning fork is detected as a change in capacitance between sensing plates built into the MEMS structure and converted to a degrees-per-second rotation rate for each of the three axis.
For a more detailed description of MEMS Gyros, see MEMS Gyroscopes and their applications. and Everything about STMicroelectronics’ 3-axis digital MEMS gyroscopes.
(Tomato, onion and tzatziki sauce extra.)