Suppose N1 is a discrete random variable equally likely to take on any integer in the set
the random variable N2 is equally likely to take on any integer in the set
, the random variable N3 is equally likely to take on any integer in the set 
(a) Find the two-dimensional joint PMF 
(b) Find the three-dimensional joint PDF 
(c) Find the marginal PDF 
(d) What are the chances that none of the three random variables are equal to 1?