  We propose an explicit spin-foam amplitude for Lorentzian gravity in three
dimensions. The model is based on two main requirements: that it should be
structurally similar to its well-known Euclidean analog, and that geometricity
should be recovered in the semiclassical regime. To this end we introduce new
coherent states for space-like 1-dimensional boundaries, derived from the
continuous series of unitary $\mathrm{SU}(1,1)$ representations. We show that
the relevant objects in the amplitude can be written in terms of the defining
representation of the group, just as so happens in the Euclidean case. We
derive an expression for the semiclassical amplitude at large spins, showing
that it relates to the Lorentzian Regge action.
