You can predict that the sequence that repeats is a,b,b
Then you can use the division by 3 to determine if a term will be a, b or c.
If the residue of the division is 1, the term will be a; if the residue is 2 the term will be b, and if the residue is 0 the letter will be b.
For example, the residue of 4/3 is 1, and the 4th term is a, while the residue of 6/3 is zero and the 6th term is b.
Now find the 19th term. If you divide 19 by 3 you get residue 1.
Then the 19th term will be letter a.