  We present a family of exact solutions, depending on two parameters $\alpha$
and $b$ (related to the scalar field strength), to the three-dimensional
Einstein-scalar field equations with negative cosmological constant $\Lambda$.
For $b=0$ these solutions reduce to the static BTZ family of vacuum solutions,
with mass $M = -\alpha$. For $b\neq0$, the solutions become dynamical and
develop a strong spacelike central singularity. The $\alpha<0$ solutions are
black-hole like, with a global structure topologically similar to that of the
BTZ black holes, and a finite effective mass. We show that the near-singularity
behavior of the solutions with $\alpha>0$ agrees qualitatively with that
observed in numerical simulations of subcritical collapse. We analyze the
linear perturbations of the threshold solution, $\alpha=0$, in the $\Lambda=0$
approximation, and find that it has only one unstable growing mode, which
qualifies it as a candidate critical solution for scalar field collapse.
