How the heck do you get three fish into one frame?..
Welcome to the forums.
You must be talking about the mission in the chamber at the end of the Dive Pool tunnel.
That one takes some patience. I position myself around the center of the chamber and do not move much except for turning around and looking up and down. You need to be ready to click the camera fast when the three fish are highlighted. I watch for fish that are already close or that are heading in my direction. If you want to move around a little, be sure to use the slow swim. I use the keyboard, so that is the shift key for me. Good luck!
Haha yeah that's indeed a tricky one, no swiming after them just stay and look, with some patients you will complete, I did it days ago too like that ;)