ClearAll[f, g, h];
 f[x_] := Piecewise[{{3, 0 < x < 1}, {-3, 1 < x < 2}}]
 g[x_] := f[Mod[x, 2]]
 h[x_] = FourierSeries[g[x], x, 30];
 Show[
  Plot[g[x], {x, -1, 3}, Filling -> Axis],
  Plot[h[x], {x, -1, 3}],
  PlotRange -> {-4, 4}
 ]

ClearAll[f, g, h, k];
 f[x_] := Piecewise[{{3, 0 < x < 1}, {-3, 1 < x < 2}}]
 g[x_] := f[Mod[x, 2]]
 h[x_] = FourierSeries[g[x], x, 30];
 k[x_] = FourierTrigSeries[g[x], x, 30];
 Show[
  Plot[g[x], {x, -1, 3}, Filling -> Axis],
  Plot[h[x], {x, -1, 3}],
  Plot[k[x], {x, -1, 3}],
  PlotRange -> {-4, 4}
 ]