First thing to know is that all three oscillators depend on +ve feedback to keep them oscillating. Some part of the output produced by the op amp is required to be fed back to its non-inverting input to keep itself in the oscillations.
The voltage level at the +ve terminal of the op amp is kept constant at half the supply, and stabilized by making use of capacitor.
A rising voltage at the output is fed back to the inverting inputs (2) that as a result cause a fall of output. This is +ve feedback and the op amp would have as table output. Also there is +ve feedback across the phase shift network.
