Can anybody explain me the hint?
If we have 3 not launched rockets, why the probability of launching each is 1/3.
There are 6 segments, that can be used
[1,1]  [2,2]  [3,3]  [1,2]  [2,3]  [1,3].

I think talk is about
Markov chain on sets of disjoint segments.
This graph is aciclic => DP is possible