xi[t_]:=  Which[t<1,-2,
  			   t<2,7t-9,
  			   t<=3,3t-1];
phi[t_]:= Which[t<1,-2t,
  			   t<2,7 (t^2)/2 - 9t + 7/2,
  			   t<=3,3 (t^2)/2 - t -9/2];
Plot[xi[t],{t,0,3}]
Plot[phi[t],{t,0,3}]