  The recently discovered vanadium-based Kagome metals $A$V$_{3}$Sb$_{5}$ ($A =
\text{K, Rb, Cs}$) are of great interest with the interplay of charge density
wave (CDW) order, band topology and superconductivity. In this paper, by
identifying elementary band representations (EBRs), we construct a two-EBR
graphene-Kagome model to capture the two low-energy van-Hove-singularity
dispersions and, more importantly, the nontrivial band topology in these Kagome
metals. This model consists of $A_g@3g$ (V-$d_{x^2-y^2/z^2}$, Kagome sites) and
$A_2''@2d$ EBRs (Sb1-$p_z$, honeycomb sites). We have investigated the Fermi
surface instability by calculating the electronic susceptibility
$\chi(\mathbf{q})$. Prominent Fermi-surface nesting peaks are obtained at three
L points, where the $z$ component of the nesting vector shows intimate
relationship with the anticrossing point along M--L. The nesting peaks at L are
consistent with the $2\times 2\times 2$ CDW reconstruction in these compounds.
In addition, the sublattice-resolved bare susceptibility is calculated and
similar sharp peaks are observed at the L points, indicating a strong
antiferromagnetic fluctuation. Assuming a bulk $s$-wave superconducting
pairing, helical surface states and nontrivial superconducting gap are obtained
on the (001) surface. In analogous to FeTe$_{1-x}$Se$_{x}$ superconductor, our
results establish another material realization of a stoichiometric
superconductor with nontrivial band topology, providing a promising platform
for studying exotic Majorana physics in condensed matter
