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.)
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Regards,
Richard Ay - Consultant