Is it correct to calculate the resultant moment as
Square root of Mx^2 + My^2 + Mz^2. I have confusion in this because Mx & Mz creates shear stress & My creates bending moment. Plz clarify

Why not learn a bit of mathematics!

If you're running a B31.1 type code, then the resultant moment is indeed SQRT(Mx^2 + My^2 + Mz^2), for the stress computation.

If you're running a B31.3 type code, then the resultant moment is indeed SQRT(My^2 + Mz^2), for the stress computation. (Here Mx was assumed to be the "torsional" moment.)