ListLinePlot[{data11, data31, data51}, PlotRange -> All, PlotTheme -> {"Monochrome"},
 PlotMarkers -> {"■", "◆", "●"}, Mesh -> 10, GridLines -> Automatic,
 PlotLegends -> Placed[{"a", "b", "c"}, {0.75, 0.3}]]

{data11, data31, data51} = Accumulate /@ ( RandomReal[#, 30] & /@ {1, {1, 2}, {2, 3}});

mesh = 10;
llp = ListLinePlot[{data11, data31, data51},  Mesh -> mesh,
   PlotRange -> All, ImageSize -> Large,
   PlotMarkers -> Thread[{{"□", "◇",   "○"}, 16}], GridLines -> Automatic,
   PlotLegends -> Placed[{"a", "b", "c"}, Right]]

ClearAll[fixMesh]
fixMesh = Module[{meshcoords = Transpose[GatherBy[Cases[#, Point[x_] :> x, All], First]],
     insets = Cases[#, GeometricTransformation[i_, ___] :> i, All],
     styles = Cases[#, {d_, _GeometricTransformation} :> d, All], add},
    add = MapThread[{#, GeometricTransformation[##2]} &, {styles, insets, meshcoords}];
    # /. {_Point -> Nothing, _GeometricTransformation -> add}] &;

fixMesh[llp]