It's possible you're thinking of a different trick, or perhaps a combination of tricks that look like a "coffin" when performed. Could you be thinking of something like a:
* "Coffin Grind" - This isn't a real skateboarding trick either. It's possible you're imagining a grind on an object that looks like a coffin.
* "Switch Backside 180 Ollie" - This trick might look like a "coffin" to some people, as the skater seems to be leaning backwards while they ollie.
* "Heelflip" - When done with a lot of body English, this trick can look like the skater is falling backwards, similar to a coffin.
If you can describe the trick in more detail, I might be able to help you find its real name or how to do it in Skate 3.