So if the girlfriend is one of the three, and not just some random fourth character (that would seriously be so aggravating) I think out of all of them Ai seems the most likely.
Megumi is firm Friendship territory, I love their back and forth, but it's way too natural for them to be some sort of cover-up. Plus the way they are acting would suck if they were secretly in a relationship as there is no warm moments between them, just "lol drink water Megumi". Plus they hardly ever walk together and Megumi is often in the background. I really
like these two as friends.
Then you have Miho who would just be super obvious if it ended up her being the girlf. If those two were trying to cover up their relationship they are doing a Poor Job, they feed each other, Nadeshiko let Miho drink her juice (lewd), and plus the "im gonna see you naked at the hot spring!" comment was just wayyy too obvious. Though Miho echoing how fun it would be to go after Ai had just said it, made it look like she was a little jealous, so ??? I kind of hope it's not Miho though, my reaction to it being Miho would just be oh ok
Ai I feel makes sense, just because the two's interactions are fairly limited which would make sense if they were trying to keep things a secret. The only reason I feel like it wouldn't be these two is the whole "friends!" sticker they had on their photos when they went photo boothing. That just feels a bit unnatural if they were together!
Still reckon its Ai and not Miho though. I would be super surprised if it ended up being Megumi