us1[x_] := UnitStep[x]

us2[x_] := -UnitStep[x]

us3[x_] := UnitStep[-x]

us4[x_] := -UnitStep[-x]

Plot[us1[x], {x, -3, 3}, PlotStyle -> {Blue, Thickness[0.01]}, 
      PlotRange -> {-1.2, 1.2}]

Plot[us2[x], {x, -3, 3}, PlotStyle -> {Blue, Thickness[0.01]}, 
      PlotRange -> {-1.2, 1.2}]

Plot[us3[x], {x, -3, 3}, PlotStyle -> {Blue, Thickness[0.01]}, 
      PlotRange -> {-1.2, 1.2}]

Plot[us4[x], {x, -3, 3}, PlotStyle -> {Blue, Thickness[0.01]}, 
      PlotRange -> {-1.2, 1.2}]

Plot[us1[x] + us2[x], {x, -3, 3}, 
      PlotStyle -> {Blue, Thickness[0.01]}, 
      PlotRange -> {-1.2, 1.2}]

Plot[us1[x] + us3[x], {x, -3, 3}, 
      PlotStyle -> {Blue, Thickness[0.01]}, 
      PlotRange -> {-1.2, 1.2}]

Plot[us1[x] + us4[x], {x, -3, 3}, 
      PlotStyle -> {Blue, Thickness[0.01]}, 
      PlotRange -> {-1.2, 1.2}]

Plot[us2[x] + us3[x], {x, -3, 3}, 
      PlotStyle -> {Blue, Thickness[0.01]}, 
      PlotRange -> {-1.2, 1.2}]

Plot[us2[x] + us4[x], {x, -3, 3}, 
      PlotStyle -> {Blue, Thickness[0.01]}, 
      PlotRange -> {-1.2, 1.2}]

Plot[us3[x] + us4[x], {x, -3, 3}, 
      PlotStyle -> {Blue, Thickness[0.01]}, 
      PlotRange -> {-1.2, 1.2}]