Help regarding question of magnetic flux in 3 phase induction motor
You need to consider the way that the three phase windings are physically positioned as well as the electrical displacements of the currents. At time zero on the diagram the flux produced by phases 2 and 3 are generally additive, but the flux produced by each is physically displaced from the phase 1 direction by 60 degrees in each direction. The amplitudes of the flux is as determined by the electrical phase displacement. It may be helpful to search for different ways that this can be diagrammed.

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I think the following diagram illustrates what the author is trying to show. One space rotation of the peak of the magnetic field corresponds to one cycle of the flux waveform. At time 0, the peak of the flux waveform is halfway between the negative direction of phase 2 and the positive direction of phase 3. This the resultant is 2 X cos 30 X cos 30 X the peak flux value.