The problem asks you to find the probability that you have die 3 if all three rolls are red.
The best way to approach this
P(D3 | R1 n R2 n R3) = P(D3 n R1 n R2 n R3)/P(R1 n R2 n R3)
find the probability of rolling three reds with die three divided by finding the probability that you roll a red 3 times.
These both can be easily done with further iteration of the process you use in 3.4b
