I've read the other answers and I believe they are better than mine for your specific problem. However I'm answering in case someone needs a generalized solution to your problem.

I recently needed to generate all permutations of the 3 separate continuous ranges [first1, last1) + [first2, last2) + [first3, last3). This corresponds to your case with all three ranges being of length 1 and separated by only 1 element. In my case the only restriction is that distance(first3, last3) >= distance(first1, last1) + distance(first2, last2) (which I'm sure could be relaxed with more computational expense).

My application was to generate each unique permutation but not its reverse. The code is here:

http://home.roadrunner.com/~hinnant/combinations.html

And the specific applicable function is combine_discontinuous3 (which creates combinations), and its use in reversible_permutation<Function, Size>::operator() which creates the permutations.

This isn't a ready-made packaged solution to your problem. But it is a tool set that could be used to solve generalizations of your problem. Again, for your exact simple problem, I recommend the simpler solutions others have already offered.

I've read the other answers and I believe they are better than mine for your specific problem. However I'm answering in case someone needs a generalized solution to your problem.

I recently needed to generate all permutations of the 3 separate continuous ranges [first1, last1) + [first2, last2) + [first3, last3). This corresponds to your case with all three ranges being of length 1 and separated by only 1 element. In my case the only restriction is that distance(first3, last3) >= distance(first1, last1) + distance(first2, last2) (which I'm sure could be relaxed with more computational expense).

My application was to generate each unique permutation but not its reverse. The code is here:

http://howardhinnant.github.io/combinations.html

And the specific applicable function is combine_discontinuous3 (which creates combinations), and its use in reversible_permutation<Function, Size>::operator() which creates the permutations.

This isn't a ready-made packaged solution to your problem. But it is a tool set that could be used to solve generalizations of your problem. Again, for your exact simple problem, I recommend the simpler solutions others have already offered.