For ~20\%~, it was enough to first check whether ~|A-B|~ is prime. If it is, then the array could simply be ~\{A, B\}~. If it isn't prime, we can check for every ~2 \le x \le 1000~ if ~x~ is prime and ~|A-x|~ is prime and ~|x-B|~ is prime. If all three hold, then the sought array is ~\{A, x, B\}~. Otherwise, there is no solution.

To obtain the full score, it was enough to note that the absolute difference between two prime numbers greater than ~2~ must be an even number. Since the absolute difference between each successive pair must be prime, we can conclude that the only numbers that could appear in the array are ~A-2~, ~A~, ~A+2~, ~B-2~, ~B~, ~B+2~ and ~2~. Out of those seven numbers, we will leave only the prime ones. Now, if we represent each number as a node in a graph and connect the two nodes if their absolute difference is prime, the task boils down to finding a path between nodes ~A~ and ~B~. This is a standard problem which we can solve using BFS.