  Family of exact spacetimes of D=3 Einstein gravity interacting with massless
scalar field is obtained by suitable dimensional reduction of a class of D=4
plane-symmetric Einstein vacua. These D=3 spacetimes describe collisions of
line-fronted asymptotically null excitations and are generically singular to
the future. The solution for the scalar field can be decomposed into the
Fourier-Bessel modes around the background solitons. The criteria of regularity
of incoming waves are found. It is shown that the appearance of the scalar
curvature singularities need not stem from singularities of incoming waves.
Moreover, in distinction to D=4 case, for all solutions with regular incoming
waves the final singularities are inevitable.
