Don't be silly. You will never get the formula in the book that way!
It was not my primary goal to find a circuit which corresponds to the "book formula". (Although I gave him a suitable circuit using an integrator stage, see my post#8).
The questioner has shown us a circuit and he gave a formula which does NOT correspond to the circuit.
Hence, in post#2 and post#8 a correct formula was given.
Regarding "silly": What did you do? You have calculated for a single unloaded RC section the frequency for a phase shift of 60 deg (by the way: rather involved; it is much simpler using the Re and Im of the denominator from 1/[1+jwRC]). In your graph you have multiplied this phase function with a factor of "3" - assuming again three decoupled RC sections (ignoring the comments in post#12 and #14).
In order not to confuse the questioner I repeat: The given formula in post#1 does NOT correspond to the shown circuit.