Three-Phase Thyristor Converter Simulation Question
Three things come to mind

1) Consider removing that high-value resistor, just use the SNUBBER options for the SCR. NOTE these don't just serve as snubber variables, but assist the simulator in solving, especially when you use inductors

2) The waveform is correct (ish, other issues, covered in #3). The reason there is humps is because it is essentially a 6pulse rectifier without any smoothing capacitance. The output voltage will follow the 3phase inputs (once phase firing has been taken into affect). 
If you were to add a 1mF capacitor on the DClink you would see something closer to DC.

3) you should control each phase with its own firing sequence. When you phase control you fire each phase w.r.t the expected zero crossing.