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By the Barkhausen Criteria, each RC makes a phase of 60°, so 60° × 3 = 180°, but with the inverting op-amp it makes another 180° which in total is 180° + 180° = 360° = 0°. Another thing is that the inverting amplifier has a gain of 29 (a bit bigger for practical op-amps). These two conditions make the oscillation. In fact, it's called a "Phase Shift Oscillator" because of the RC components.
