My reasoning for that would be the chest size discrepancy between third girl from the past and Hiba in the present. But I didn't count her out just yet because
this panel shot of her (pay attention to her hair) is not too dissimilar to the
one in the picture, and the breast size difference could be explained either by pads, the picture was taken at a later date, or her choice of clothes makes her look slimmer than she really is.
Yeah, the bust size is one of the major reasons I don't think that it's Hiba in the photo, the girl in the photo seems to be even larger than Luna. In the present time in the manga, I don't think that would be easy to disguise or explain away.
Other than that, even though we've only seen Hiba once in the flashback, the style of clothing seems a bit too different from what she would wear (conjecture based on only what we've seen). Combined with that fact that the third girl is standing on the other side of Naoto when you would expect her to be standing with her friend if it was Hiba, it raises some doubts about it being her (a bit of a reach).
Unfortunately, long black hair is the norm, so it's pretty hard to use that to try and make the connection.
I'm still thinking the third girl (if it isn't Hiba) was older than the rest of them (maybe late teens, early 20's) and she is going to be some kind of milf character, or even a teacher (who knows at this point).