  We consider the sub-Riemannian $3$-sphere $(\mathbb{S}^3,g_h)$ obtained by
restriction of the Riemannian metric of constant curvature $1$ to the planar
distribution orthogonal to the vertical Hopf vector field. It is known that
$(\mathbb{S}^3,g_h)$ contains a family of spherical surfaces
$\{\mathcal{S}_\lambda\}_{\lambda\geq 0}$ with constant mean curvature
$\lambda$. In this work we first prove that the two closed half-spheres of
$\mathcal{S}_0$ with boundary $C_0=\{0\}\times\mathbb{S}^1$ minimize the
sub-Riemannian area among compact $C^1$ surfaces with the same boundary. We
also see that the only $C^2$ solutions to this Plateau problem are vertical
translations of such half-spheres. Second, we establish that the closed
$3$-ball enclosed by a sphere $\mathcal{S}_\lambda$ with $\lambda>0$ uniquely
solves the isoperimetric problem in $(\mathbb{S}^3,g_h)$ for $C^1$ sets inside
a vertical solid tube and containing a horizontal section of the tube. The
proofs mainly rely on calibration arguments.
