3-dimensional Levi-Civita metrics with projective vector fields

Projective vector fields are the infinitesimal transformations whose local flow preserves geodesics up to reparametrisation. In 1882, Sophus Lie posed the problem of describing 2-dimensional metrics admitting a non-trivial projective vector field, which was solved in recent years. In the present paper, we solve the analog of Lie's problem in dimension 3, for Riemannian metrics and, more generally, for Levi-Civita metrics of arbitrary signature.