A pond has $r$ red fish, $b$ blue fish, and $g$ green fish. Let $R$ be the number of red fish, $B$ be the number of blue fish, and $G$ be the number of green fish in a random sample size of $N$. What is the joint probability mass function of $R$, $B$, and $G$?
I understand that $G$ should be hypergeometrically distributed since only a certain number of the fish are green, but how do I construct a joint distribution?