Froakie = Water/Fighting Fennekin = Fire/Psychic Chespin = Grass/Dark Done. Everyones happy. And they all have Sp. ATK of over 300 and speed over 250.
this makes the most sense because of Psychic>Fighting>Dark hence the first time all starters would actually be even. Unless I am mistaken.
Also, added tidbit, if it was Water/Fighting, Fire/Psychic, and Grass/Flying then that would kind of make Froakie screwed and useless against either other starter so you would have to change Fennekin to something else, Who knows what else.