Answer
$(3,0,0)$.
Work Step by Step
Since $\rho=3, \, \theta=0 \, \phi=\frac{\pi}{2}$ and
$$ x=\rho\cos \theta\sin \phi,\quad y=\rho\sin\theta\sin\phi, \quad z=\rho\cos \phi,$$
then we have
$$ x=3\cos 0\sin \frac{\pi}{2}=3, $$
$$ y=3\sin 0\sin \frac{\pi}{2}=0, $$
$$ z=3\cos \frac{\pi}{2}=0.$$
Hence, the rectangular coordinates are $(3,0,0)$.
