AC-AC Converter 3 phase, Line to line voltage calculation
Start by minimizing the problem to firstly this: -



Then, because the negative half cycle is a mirror image of the positive half cycle, it will have the same RMS value, thus we end up with only needing to analyse this section to get RMS: -



Then it breaks down into 4 sections that you need to find the square of the RMS for each: -



So, find those individual squares then multiply each by their time duration and the final RMS value of your waveform is this: -

$$\sqrt{\dfrac{d_1V_1^2 + d_2V_2^2 + d_3V_3^2 + d_4V_4^2}{d_1+d_2+d_3+d_4}}$$