ClearAll;
f[t_]:= Which[t<=2,3+2t,
              t<=4,13-3t,
              t<=5,t-3,
              t<=7,2];
L[t_]:= Which[t<=10/3,3,
              t<=4,13-3t,
              t<=7,1];
U[t_]:= Which[t<=2,3+2t,
              t<=7,7];
p1 = Plot[f[t],{t,0,7},AxesLabel->{"t","y"},PlotLabel->"y = f(t)",
	PlotRange->{{0,7},{0,7}}];
p2 = Plot[L[t],{t,0,7},AxesLabel->{"t","y"},PlotLabel->"y = L(t)",
	PlotRange->{{0,7},{0,7}}];
p3 = Plot[U[t],{t,0,7},AxesLabel->{"t","y"},PlotLabel->"y = U(t)",
	PlotRange->{{0,7},{0,7}}];
Show[GraphicsArray[{{p1},{p2},{p3}}]]