The objective is to allow for the fact that it is quite possible for someone to accidentally come across such a video, be curious and watch it one time and perhaps a second time. I am not pretending that there is something magical about the number three.
This is an attempt to capture repeated viewing, which may suggest that the intent is not innocent. Of course, should the Bill become an Act of Parliament and someone is prosecuted under this law, that decision would be made by the police, based on evidence and working with the Crown Prosecution Service. As with other criminal offences of this type, the CPS would use its judgment to decide whether it is in the public interest to prosecute.