f[u_] := 
        Which[-∞ < u < 0, 0, 0 < u < 1, 1, 1 < u < 2, 0.5, 
          2 < u < ∞, 0];
Plot[f[u], {u, -5, 5}, AxesLabel -> {"u", "g(u)"}, 
      PlotStyle -> {Thickness[0.01], RGBColor[0, 1, 0]}, 
      PlotRange -> {{-5, 5}, {0, 3}}]

Plot[f[-u], {u, -5, 5}, 
      AxesLabel -> {"u", "g(-u)"}, 
      PlotStyle -> {Thickness[0.01], RGBColor[0, 1, 0]}, 
      PlotRange -> {{-5, 5}, {0, 3}}]

Plot[f[3 - u], {u, -5, 5}, 
      AxesLabel -> {"u", "g(3-u)"}, 
      PlotStyle -> {Thickness[0.01], RGBColor[0, 1, 0]}, 
      PlotRange -> {{-5, 5}, {0, 3}}]