I remember a thread on here at the time, there were people absolutely determined to think it was some sort of triple murder. Usually most likely explanation is the right one. It would be very unlikely that they would all accidentally fall in and be unable to get out/shout for help, particularly as they hadn't been drinking or anything, but even that was more likely than all three being randomly killed, presumably by at least two or three people working together, without anyone noticing.
The fact that the family couldn't find any photos of them as adults and used AI to mock one up of what they would look like based on childhood photos was also very odd. Of course I feel for their surviving family but it seems like there was probably a lot going on under the surface there for all 3 to feel suicide was the only way out.