I think she managed to look plenty serious in the image-set for Only You, though? Some hated on her hairstyle (I loved it) but for some cuts she definitely looked serious enough. Not really cool or anything, but just... serious. And if you watch her during those rehearsal vids we got earlier she definitely has that serious face a lot, it's just that when attention gets to her she goes all %D
To be fair though, we only have Only You to go by so far as everything else they've done have been happy genki stuff. And Only You was shot like, in May or something? So by now I definitely think she'd be able to pull off a cool-thing.
While Masaki and Haruka definitely would not be able to. Even if they'd have serious/empty expressions, their faces are still very childish. Which...makes sense, since they're 12.