I've used the 5x tele on the PM 15 and it is surprisingly stable when shooting video. Pretty amazing tech. However, I agree that I miss the 3x support, especially in Portrait or Cinematic mode. I believe Apple will "solve" this when they add a fourth lens to the iPhone Ultra. (If they truly are adding that shutter button, you know this is eventually coming.)Honestly, I'm glad I didn't get the 15PM and got the pro instead. The 3x is long enough and 5x would be too long for most uses. Plus, for video 3x is already hard to keep stable, and 5x is just going to be a shaky mess.
They should just bite the bullet and go full Samsung Galaxy Camera style with a proper telescoping zoom lens on the back of the camera. Just make a version that is a camera that also happens to be a phone.