I think the line is crossed when there is harm to a child. In the first example (actual pictures), a child has been exploited and therefore harmed. Thats not the case in the other two scenarios.
However an interesting question arises with the second and third scenario. If a pedophile uses these simulated images, does it increase or decrease the possibility of them commiting a crime. You could argue that the images might cause a pedophile to desire the real thing even more, by the temptation of what they are seeing. You could also possibly argue that their self-gratification to these images might replace an urge to actually find a real child.
I’m not sure what that answer is though because I have no clue how the mind of a pedophile works. I would imagine it happens both ways. Some are more likely to act because of these images and some, less so.
Having said that, I see another danger in the simulated child-porn scenario, whether with real models or digital creations. It could be used to lure a kid into thinking that it is ok to have sex with an adult. “See, they are having fun.” In that respect, I think it is better to err on the side of caution, and not have any of these images be allowable. The Supreme Court appears to disagree however.