Well, the first one is easy to find, since it is literally over half of fanmade pictures, so there are plenty of examples.

The second one is also abundant, mostly through "adult" Flash Games. The flash animation allows art style replication to be quite easy.

As for the third one..?

Not quite what it says, but not only are the drawers of Highschool of the Dead the same ones to have drawn actual hentai before, but also, as you can see, HSotD's OVA is a lot more explicit than the anime itself, as well. So, technically counts.

Spot on. Screw those shitty Osomatsu-san doujinshis plagued with a bishonen artstyle, I only fap to the original artstyle.