There are three independent rings, each with 6 options. So $6^3$ is the right answer (the order matters). This assumes the rings are fixed and known in advance.
But this is not among the options.
Your answer (nr 3) is the number of ways we can pick the letters for each ring first (so start with blank rings), and then times $6^3$ for each combination.
But this overcounts. The combination ABC could occur in many different ways.
The net effect of picking rings and letters for each is that every combination of three different letters could be a combination for lots of ring configurations so the answer would then $26 \cdot 25 \cdot 24$.
So if the rings are fixed, the answer is $6^3$, if they're configurable, the answer is $26 \cdot 25 \cdot 24$. The question is unclear, IMHO, and not very good.