  We have generalized the results of the previous work [arXiv:2302.12209] to
the case of three-dimensional (3D) spacetime with two spatial and one temporal
coordinates. We have found that the flat Minkowski 3D spacetime is
"well-stitched", which means that it possesses a structure described by 24
causal relations between 12 events. We have proved that a 3D spacetime is
"well-stitched" if and only if it is conformally flat. The concept of a
"well-stitched" spacetime does not rely on metrical information about lengths,
times, etc., and does not belong to the metric geometry, but rather to geometry
of incidence. We therefore have "translated" an important concept of a
conformally-flat spacetime from the "metric" language of Riemannian geometry to
the "non-metric" language of the geometry of incidence. The results of this
paper provide a tool for detecting the curvature of the 3D spacetime on the
basis of causal relations only, without any measurement instruments like rulers
and clocks, provided that the spacetime is not conformally flat.
