  We theoretically study the competition among different electronic phases in
molecular conductors $\kappa$-(BEDT-TTF)$_2$X. The ground-state properties of a
3/4-filled extended Hubbard model with the $\kappa$-type geometry are
investigated by a variational Monte Carlo method. We find various competing
phases: dimer-Mott insulator, polar charge-ordered insulator, 3-fold
charge-ordered metal, and superconductivity, whose pairing symmetry is an
"extended-$s$+$d_{x^2-y^2}$"-wave type. Our results show that the
superconducting phase is stabilized not on the verge of the Mott
metal-insulator transition but near charge order instabilities, clearly
indicating the importance of the intradimer charge degree of freedom and the
intermolecular Coulomb interactions, beyond the simple description of the
half-filled Hubbard model.
