  In this article, several cohomology spaces associated to the arithmetic
groups $\mathrm{SL}_3(\mathbb{Z})$ and $\mathrm{GL}_3(\mathbb{Z})$ with
coefficients in any highest weight representation $\mathcal{M}_\lambda$ have
been computed, where $\lambda$ denotes their highest weight. Consequently, we
obtain detailed information of their Eisenstein cohomology with coefficients in
$\mathcal{M}_\lambda$. When $\mathcal{M}_\lambda$ is not self dual, the
Eisenstein cohomology coincides with the cohomology of the underlying
arithmetic group with coefficients in $\mathcal{M}_\lambda$. In particular, for
such a large class of representations we can explicitly describe the cohomology
of these two arithmetic groups. We accomplish this by studying the cohomology
of the boundary of the Borel-Serre compactification and their Euler
characteristic with coefficients in $\mathcal{M}_\lambda$. At the end, we
employ our study to discuss the existence of ghost classes.
