CMOS frequency of oscillation
Because the delay around the loop must be 360 degrees (why? Because the output at the junction of R3/C3 is identical to the input of the first inverter), it's clear that the phase shift of each section must be 120 degrees. And since this represents the sum of the RC phase shift plus the 180 degree phase shift supplied by the inverter, the RC phase shift must be 60 degrees.

Since phase shift angle is$$\phi = tan^{-1}\frac{1}{2\pi fRC }  $$ set the angle to pi/3 and solve.

EDIT -

Please note that this approach only works for phase shift oscillators with a small number of stages. If you have a lot of stages, each RC spends most of its time at one level or the other, and the concept of phase really won't apply well. In this particular case, you can see that the RC waveform is not too far off from a sinusoid, so the linear criteria work pretty well. They'll work even better, of course for a sinusoid, but beggars can't be choosers.

END EDIT