3PPKH+K does more damage than either of the other two, but, because the last kick is slow, it tends to whiff when the K2K and 6PP enders still connect.
K2K does very slightly more damage than 6PP. And other than that, it slams the opponent more far away. So if you're fighting someone that you definitely don't want to be extremely close to you all the time and want them to close the gap, like Sarah or Helena or Pai, the K2K ender is better than 6PP since the opponent is pushed more far away.
Close to walls, 6PP is superior since the first K will slam the opponent into the wall, ruining your juggle. And don't use it against the likes of Jann Lee, who often simply like to space all day.