As the RC HPF phase shift of 3x 60 degree= 180 degree phase shifters is RC= 100ns =T for the 3rd stage, in order to satisfy the Barkhausen Criteria threshold of instability (oscillation) the positive feedback for a sine wave must be Av=1 @ 360 deg.
Tricky Tacky Details
However you will notice that each stage is not independent but “apparently” (pun intended) has either an load or source impedance that is not 0 like the 1st stage comes from 0 Ohms. So the phase shift is not exactly = 3x RC but slightly higher. (If R was 1,10,100x and C= 1,0.1,0.01x then it would almost be isolated but with same RC=T. Nevertheless an approximation will do using a pot to expect >6dB per stage and not quite -60 deg per stage ( and thus more gain compensation)
Nevertheless intuitively you know at 45 deg the attenuation of an RC =T network is when | Xc(f)| = R and -6 dB and to get 60 deg, a higher frequency. Is required with more attenuation which you can solve with trig.
Thus as all good 1st yr Physics students shud know in lab work, how close does your computed values match your result with the tolerance stack up of your components in order to validate your theory. Getting it to work is not enough. You must learn to analyze all the sources of error.
There is an expression for “goodness” on the error in order to validate your theory of prediction and apparent loading is a predictable error. (Besides the brain farts)
This is more important than the low Q phase shift Oscillator design.
Q is ratio of the fo/ BW - 3dB of this low Q (high phase noise, an fo tolerance).
These have nothing to do with Op Amp experience using any general purpose part, however high speed ones used for low frequency with a gain >= 1e6 may have a tendency to oscillate at their own max frequency from parasitics with the added positive feedback and another zero at Av=1 internally at GBW/1.
So in future if you see any unstable ringing from step response, look for parasitic coupling to positive feedback or mismatched driver to cable impedance.
One final thing is as the Op Amp hits saturation the gain goes from 1e5 (ish) to zero , yes that’s “0”, so limiting will not change with a small change in input (gain=0). But before this cliff, the gain will reduce enough to soft limit the loop gain with the correct value of pot to affect the total harmonic distortion value or THD and yield a decent sine wave. But in future don’t use this design, there are better ones.