  An $i$-packing in a graph $G$ is a set of vertices that are pairwise distance
more than $i$ apart. A \emph{packing colouring} of $G$ is a partition
$X=\{X_{1},X_{2},\ldots,X_{k}\}$ of $V(G)$ such that each colour class $X_{i}$
is an $i$-packing. The minimum order $k$ of a packing colouring is called the
packing chromatic number of $G$, denoted by $\chi_{\rho}(G)$. In this paper we
investigate the existence of trees $T$ for which there is only one packing
colouring using $\chi_\rho(T)$ colours. For the case $\chi_\rho(T)=3$, we
completely characterise all such trees. As a by-product we obtain sets of
uniquely $3$-$\chi_\rho$-packable trees with monotone $\chi_{\rho}$-coloring
and non-monotone $\chi_{\rho}$-coloring respectively.
