In case of 3 phase system , there is no need to calculate triple harmonics. It will be eliminated from line - line voltage. Firstly determine, V1, V5, V7, V9, V11, V13,......up to V49, Using SHE equations after calculating switching angles. Then, use voltage THD equations to compute THD. My suggestion is that it would easier if you write MATLAB programming for voltage THD up to 49 th harmonics. In this case, for different values of modulation index, voltage  THD can be easily computed.

Dear Abdul Jabbar,

It would be better to use FFT option of powergui available in MATLAB Simulink.

You will be able to see a clear picture of each frequency component as well as the THD value.

Regards

If you have done the hardware development then you need to obtain the waveform of the input current through an oscilloscope then you can go through this video 

https://www.youtube.com/watch?v=dM1y6ZfQkDU&list=PLjvrdW1oOwtnlykQHDunDFdApi9e1DEkZ&index=1

Dear Abdul jabbar Khan,

From my humble perspective, the simple way is to use powergui FFT analysis tool available in MATLAB/Simulink. The FFT tool is very convenient and easy to use. Besides, it provides detailed information on each harmonic component where you can actually see which harmonic component contribute most to the THD. 

If you are designing your multilevel inverter using simulink block then it will be very convenient because you just need to run the simulation and then check the THD of your multilevel inverter output using the FFT option in the powergui.

If you are not using MATLAB/SImulink, then you can save the output of your multilevel inverter in Microsoft Excel file and export the file to simulink by using "signalbuilder" block. The same method can be applied for your hardware development as well because  signal captured using oscilloscope can also be saved in Excel format. 

Hope the method is helpful in your case.