  We are concerned with the following coupled Schr\"{o}dinger system with Hardy
potential in the critical case \begin{equation*} \begin{cases} -\Delta
u_{i}-\frac{\lambda_{i}}{|x|^2}u_{i}=|u_i|^{2^*-2}u_i+\sum_{j\neq
i}^{3}\beta_{ij}|u_{j}|^{\frac{2^*}{2}}|u_i|^{\frac{2^*}{2}-2}u_i, ~x\in
\mathbb{R}^N, u_i\in D^{1,2}(\mathbb{R}^N),\,\, N\geq 3,\,\, i=1,2,3,
\end{cases} \end{equation*} where $2^*=\frac{2N}{N-2}$, $\lambda_i\in
(0,\Lambda_N), \Lambda_N:= \frac{(N-2)^2}{4}$, $\beta_{ij}=\beta_{ji}$ for $i
\neq j$. By virtue of variational methods, we establish the existence and
nonexistence of least energy solutions for the purely cooperative case
($\beta_{ij}> 0$ for any $i\neq j$) and the simultaneous cooperation and
competition case ($\beta_{i_{1}j_{1}}>0$ and $\beta_{i_{2}j_{2}}<0$ for some
$(i_{1}, j_{1})$ and $(i_{2}, j_{2})$). Moreover, it is shown that fully
nontrivial ground state solutions exist when $\beta_{ij}\ge0$ and $N\ge5$, but
{\bf NOT} in the weakly pure cooperative case ($\beta_{ij}>0$ and small, $i\neq
j$) when $N=3,4$. We emphasize that this reveals that the existence of ground
state solutions differs dramatically between $N=3, 4$ and higher dimensions
$N\geq 5$. In particular, the cases of $N=3$ and $N\geq 5$ are more complicated
than the case of $N=4$ and the proofs heavily depend on the dimension. Some
novel tricks are introduced for $N=3$ and $N\ge5$.
