I have expressions for marginal probability distributions p(a) and p(b). The joint probability distribution p(a,b), if I understood correctly, can be obtained by the product p(a)*p(b). I need to find the conditional probability P(Y|a,b). What information about Y is needed for doing this? If the probability distribution of Y is simply given as p(Y), what would be the expression for conditional probability?
One way or another, the joint distribution of $(Y,a,b)$.