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First of all, P is in itself a graphics object. Writing plot3d(P) does not make much sense.

Secondly, f is a one-variable function so that plot(f) is a 2 dimensional picture (that works). But then the addition plot(f)+plot3d(P) is trying to concatenate a 2-dimensional and a 3-dimensional picture that looks weird.

Now, to draw a curve in 3d you can for example use parametric_plot3d.