PMSM motor current calculated by D axis and Q axis current
If you have your \$i_d\$ and \$i_q\$ currents, you can perform the inverse Parks-transformation (thus you obtain the \$i_\alpha\$ and \$i_\beta\$ currents) and subsequently the inverse Clarke-transformation. 
Then you have the three sinusoidal currents \$i_a\$, \$i_b\$, and \$i_c\$, which have the same amplitude and a phase difference of 120° to each other. 

The RMS value of a sinusoidal signal is easy to derive, and it is given by \$\frac{A}{\sqrt{2}}\$, where \$A\$ is the amplitude of the signal.