The complex numbers can be represented either by Cartesian coordinates x+j y, or in a polar form z= a exp j theta, with a is the magnitude and theta is the phase.

Concerning the three phase source they can be best expressed in  a polar form V, Vexp j120 degree, Vexp -j120 degrees. They can be converted into the Cartesian form as:

z = a cos theta + j a sin theta.

Best wishes 

I would suggest using a basic "gain" block, setting the gain to "-0.5-sqrt(3)/2*i"

I agree with Dr. Wolfgang, you can use the gain block in simulink and can define its value either in the simulink itself or in matlab workspace.

Thanks for the inputs, I have tried to use the basic gain block and entered the value -0.5-sqrt(3)/2*i, then I am passing a sinusoidal signal of frequency 314 rad/s, and then connecting a scope; the error "Complex signal mismatch. Output port 1 of 'untitled/Gain' is a signal of numeric type complex. However, it is driving a signal of numeric type real" comes.

To over come the same I have used a complex to real-imag block.

I have enclosed the snapshot of the input and output signals. Should I take the real part and consider as the output a2Vb ?