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)

[![enter image description here][1]][1]

[![enter image description here][2]][2]

[![enter image description here][3]][3]

And this one for another "drive" sequence.

[![enter image description here][4]][4]


  [1]: https://i.sstatic.net/xTl39.png
  [2]: https://i.sstatic.net/PHTzl.png
  [3]: https://i.sstatic.net/MG1Pj.png
  [4]: https://i.sstatic.net/WBc6i.png