Each set of games had there own theme or ideas to them.
This time I think the games are focused on individuality based on the starters.
Froakie represents the original starters as in being amphibious like the original trio.
Chespin represents the future of starters as in pokemon that more creative in design with a spring of generic cuteness.
Fenniken represents the present, being a generic "cute" animal that appears in all of the games.
Design wise the legendary also seem completely different from each other.
Odds are the third member will be able to transform between the two.