work out the probability of the 3rd generation containing 1,2,3,... pairs of adults.
Let the number of pairs of adults in generation 3 be X.
Let the size of generation 4 be Y.
Use this identity: P(y=0) = P(y=0 and x=1) + P(y=0 and x=2) + p (y=0 and x=3) ..... up to the maximum possible value of x.
Check you understand why this works.