Answer
The rectangular coordinates are $(0,0,3)$.
Work Step by Step
Since $\rho=3, \, \theta=\pi, \, \phi=0$ and $$ x=\rho\cos \theta\sin \phi,\quad y=\rho\sin\theta\sin\phi, \quad z=\rho\cos \phi,$$ then we have $$ x=3\cos \pi\sin 0=0, $$ $$ y=3\sin \pi\sin 0=0, $$ $$ z=3\cos 0=3.$$ Hence, the rectangular coordinates are $(0,0,3)$.
