3? That sounds crazy to me. I don’t have children, but I have babysat children and would never leave a 3 year old alone in a bathtub. At age 5 I might leave for a minute to take care of something, but be talking to them the whole time to make sure the child is responding and ok. If it was two children I might feel a little more secure if I can hear them talking and playing in the water, and there is a buddy system so to speak. That is assuming the children don’t tend to want to kill each other, but, would still need to be within ear shot. But, not three. Three is too young.
This sounds like a case of, nothing has ever happened before so it is ok. like on Q’s when people say I leave my door unlocked all the time and have never been robbed. Maybe the bad thing is not likely to happen, but if it does it will be awful. A toddler drowning in a tub is really so bad, even if it only happens one in 300,000 of children left alone (made up number) the result is too devastating to risk it in my opinion.