If ya look close in the trailer, you can see Fenniken use what looks like Psybeam, Chepsun use what I believe to be Furry Swipe, and Foroky looks like he might have used some type of fighting move. This leads me to deduce that each Pokemon's final evolution will each be multitype, Fenniken being Fire Psychic, Chepsun being Grass Dark, and Foroky being Water Fighting. This also gives a type advantage wheel that works in reverse so that each starter may have a chance against one another.
Yeah, honestly I really doubt they can shift Fenniken's form and more so that it actually looks like it could work as one. Not to mention have a reverse type triangle of Fighting/Psychic/Dark working along with with the classic Water/Fire/Grass makes that starters a much more balanced bunch. And I hope it is to!