Absolute value of complex numbers in 3D plot

edit

(In your example, you could just replace abs(i*omega) by abs(omega), but I'm sure you gave a simpler example than the examples you really care about.)

It often helps to use a lambda function for plots, as in:

plot3d(lambda x, omega: 1+x+abs(i*omega), (x,0,5), (omega,0,5), viewer='tachyon')

I think the explanation is that functions applied to symbolic expressions often misbehave, while the lambda trick makes sure the functions only get applied to numerical input for plotting.

EDIT: In the new problem described in your edit, you ask for a 3d plot of a function of omega and V_C, but this function depends only on omega, and not at all on V_C, so a 2d plot would suffice. But maybe that's again just because you used a simplified version of your input, for the sake of the question.

One way to make things work is to make 'G_do_ideal_qn' a function rather than just an expression. Below I'm shortening a tiny bit the names and the values just to make things fit on one line.

sage: G_do(om,V) = -31.*(0.03*I*om - 359.)/(-0.0003*om^2 + 1.03*I*om + 180.)
sage: plot3d(lambda om, V: 20*log(abs(G_do(om,V)), 10), (om, 10, 100e3), (V, 0, 400))