There are likely several options. If you mean on one set of axes, the hold function is likely best. If you want each on separate axes in the same figure, use subplot.
Illustrating:
x = linspace(-5, 5);
y1=x.^2;
y2=-5*x+2 ;
y3=4*ones(size(x));
figure(1)
plot(x, y1)
hold on
plot(x, y2)
plot(x, y3)
hold off
grid
figure(2)
subplot(3,1,1)
plot(x, y1)
grid
subplot(3,1,2)
plot(x, y2)
grid
subplot(3,1,3)
plot(x, y3)
grid
