The tree spanner problem for a graph $G$ is as follows: For a given integer $k$, is there a spanning tree $T$ of $G$ (called a tree $k$-spanner) such that the distance in $T$ between every pair of vertices is at most $k$ times their distance in $G$? The minimum $k$ that $G$ admits a tree $k$-spanner is denoted by $σ(G)$. It is well known in the literature that determining $σ(G)\leq 2$ is polynomially solvable, while determining $σ(G)\leq k$ for $k\geq 4$ is NP-complete. A long-standing open problem is to characterize graphs with $σ(G)=3$. This paper settles this open problem by proving that it is polynomially solvable.