"Letters from the Middle of the Sea"

I think that's the translation?

Okkkk so swimsuit set, looks like it could be an underwater cave themed-like set, judging from the background

OH!!! I JUST NOTICED, THE STAMPS (I think)!!!

So the darker blue is obviously Tae, light blue looks like Hina, green could be Maya or Rokka, reddish-orange(?) looking like Kasumi or Hagu, aaand I'm drawing a blank on the farthest one, I think it's probably Tsukushi?

Hina's probably the other 4* seeing her stamp with Tae's

Seeing the colour of the text, Maya/Rokka's the gacha 3*

plz let this be powerful plzzz