Yeah there are flat 3's but most are just "rodeo 3's" or off axis backflip/loop. Here's a video of Mike Henitiuk throwing one of the most legit flat 3's out there, and it's in the moguls. The goal is to not go inverted, Henitiuk does a little, but if done right it's one of the toughest tricks out there.
That would be a misty 5.
But yeah "flat" 3's do exist, but they are mostly just rodeo 3s. They are halfway in between backflip and lincoln loop. Warnick does a pretty dank rodeo 3 in the 4bi9 gets aspen video. Check it out at 1:08 http://www.4bi9media.com/08/view_video.php?name=4Bi9%20Gets%20Aspen