[R] How to plot three graphs on one single plot?

[khazaei at ceremade.dauphine.fr]

Hello alla

I have three kinds of information (x,y1),(x,y2),(x,y3) such that x and
y1,y2,y3 are the vectors with the same dimention.
Now I want to plot these three class af information on one plot.
thank you for your help.
khazaei