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 $χ_ρ(G)$. In this paper we investigate the existence of trees $T$ for which there is only one packing colouring using $χ_ρ(T)$ colours. For the case $χ_ρ(T)=3$, we completely characterise all such trees. As a by-product we obtain sets of uniquely $3$-$χ_ρ$-packable trees with monotone $χ_ρ$-coloring and non-monotone $χ_ρ$-coloring respectively.