How to explain a language with modulo conditions is regular?
The easiest way is by giving a regular expression for $L$:
$$
L = (a+b+c)^2((a+b+c)^3)^*a.
$$
Here $(a+b+c)^2$ is a shorthand for $(a+b+c)(a+b+c)$, and $(a+b+c)^3$ is a shorthand for $(a+b+c)(a+b+c)(a+b+c)$.

Other ways described in the comments:


Give a DFA (6 states) or an NFA (4 states) for $L$.
Decompose $L = L' a$ and give a DFA (3 states) or an NFA (3 states) for $L'$.