  A three-orbital model derived from the two-dimensional projection of the $ab$
$initio$ Hamiltonian for alkaline doped fullerene A$_3$C$_{60}$ with A=Cs,Rb,K
is studied by a variational Monte Carlo method. We correctly reproduce the
experimental isotropic s-wave superconductivity around the $ab$ $initio$
parameters. With narrowing the bandwidth, the transition to an insulator is
also reproduced, where orbital symmetry is found to be spontaneously broken
with emergence of an excitonic Mott insulator for two orbitals and an
antiferromagnetic insulator nearly degenerate with a spin liquid for the third
orbital. The superconductivity is a consequence of exciton melting.
