- b c - - b b - a a b c

We start the series with a,b,c as the first 3 terms.

For the next 3 terms, we start with the last term i.e 'c' of the previous series(a,b,c in his case), and continue with the next 2 terms in circular fashion (a,b,c,a,b,c). So the second set of 3 terms comes out as c,a,b.

Continuing in a similar way, the next 3 terms are b,c,a, and the subsequent terms are a,b,c.

Hence, the final series turns out as a,b,c,c,a,b,b,c,a,a,b,c.