All three lines are being plotted. The issue is that the green line goes to huge values, so you don't see the separation between the other two. Here, I've plotted over a much smaller range, so you see that:
t = [-1.5:0.01:1.5];
x = t.^3 - 2*t + 9;
y = 6*t.^5 - t;
z = t.^2 + 7;
plot(t,x,'r:')
hold on
plot(t,y,'g--')
hold on
plot(t,z,'b-.')
hold off
FYI, you don't need to repeatedly use hold on ...
t = -1.5:0.01:1.5
x = t.^3 - 2*t + 9;
y = 6*t.^5 - t;
z = t.^2 + 7;
figure
hold on
plot(t,x,'r:')
plot(t,y,'g--')
plot(t,z,'b-.')
hold off
