Self-similar integer sequences are certain sequences that can be reproduced by extracting a portion of the existing sequence. See the OEIS page for more information.
In this problem, you are to check if the sequence is self-similar by every third term. The problem set assumes that you start with the first element and then take every third element thereafter of the original sequence, and compare that result to the first third of the original sequence. The function should return true if the extracted sequence is equal to the first third of the original sequence.
Since seq_every_third = seq_orig_first_third, the set is self-similar.