number of ways in which it is possible to make a selection from m+n+p=N things,where p are alike of one kind, m alike of another kind and n alike of another kind,taken r at a time is given by co-efficient of x^r in the expansion of

(1+x+x^2+x^3+......+x^m)(1+x+x^2+.....x^n)(1+x+x^2 +x^3+.......x^p).

How is this established?Explain me the Q.E.D.