I can cork 3 but not 7. the rotation in cork 3s are not anything like other corks. Just carve really hard, commit and try to get your feet over your head mid air in the rotation. It's a lot more easy to learn them on a steep jump because then you are almost corked when leaving the kicker!
Good luck with it and soon you will have the best trick in the world stacked up in your trickbag! ;)