Here what one get when it is a 3-phase circuit.
Ean ... does not seem to be what is in the book ...
I did 3 simulations.
One with only 1 reverse SCR fired, Second with 2 SCR fired, Third with 3 SCR fired.
One can see that waveforms are not the same ... (unless error.s)
And this one for another "drive" sequence.
Here is the calculation of "RMS voltage" versus "phase angle" drive.