Each represents one of the common races of the world – human, elf, and forestkin, respectively. Each also has their own class, as well as other unique traits. For example, Sable has a cock, which has certain gameplay implications, while Lilina’s status as Princess means certain enemies will make capturing her a priority.
These three will, on order of the Giannan Parliament, be traveling across the world seeking six jewels, as well as six tomes to unlock the power of those jewels. That’s the main questline, anyway – there are basically no restrictions on where and what you can do!