Thanks to Instructor Matthew Harkins for providing the detailed count breakdown.
Introduction: A Newcomer level move for Foxtrot
Frame: Standard. Count: 1+2+3+4+.
Leaders and Followers: Leaders and Followers move the same.

Instructions (Leader & Follower Perspective)
For Three Step, your left foot is the one that moves forward. This will always hold true, for you cannot do feather step with your left nor three step with your right. The two main aspects this page will focus on is how to move forward and backwards with three step. Similar to feather step, there's a specific count, so read away.

Basic Three Step is done Heel, Heel, Toe. As in, your left foot steps forward, landing with the heel. Your right foot then moves forward, landing on its heel. Only after accomplishing this do you then have your left leg swing forward, landing on the toes. The reason why Three Step is HHT while Feather Step is HTT is because with feather step, you're stepping outside partner. This means you can comfortably step on the second step with your toes and your partner can compensate. But with Three step, you're stepping between your partner's legs. If you try to do the second step as a toe, you're going to jam straight up into your partner. And since the leader is usually the one stepping forward and is often taller than the follower, this is an extremely uncomfortable position for your partner. Not cool.

Going Forward
1: Start with your feet together. On this count, release your weight. Your left foot will consequently slide forward, left knee more straight than bent. Your right knee will be bent. Keep in mind that your right knee should also go slightly forward. Similar to Feather Step, your entire body is moving forward, not just your legs.
And: Extend both legs to split weight position, left foot in front.
2: Begin taking left knee forward.
And: Collect feet under body.
3: Staying low, push with your left leg, extending with your free right leg forward to split weight position.
And: Rise and bring knee forward on right leg. This will collect your feet under your body.
4: Using toe pressure on right foot, allow left foot to swing into split weight position.
And: Collect feet under body and lower.

Going Backwards
We will assume your partner is executing Three Step going forward. Going backwards is similar for both left and right sides, unlike say going forward (HTT for right side, HHT for left side).
1: Weight lowering. Right foot should be slightly back. Similar to moving forward, where your weight drop is moving forward and down, your weight drop here is moving backwards and down. Your body should not be left behind while your legs move to where they go. Instead, body and legs should move as a unit.
And: Split weight position. Even trickier than when going forward, but it must be done. Ball of your right foot, heel of your left foot.
2: Begin taking your left leg backwards. Upper body should be over your right foot at this point.
And: Collect feet under the body. Weight is on your right leg as you prepare to drive left foot backwards.
3: Split weight position, This time, right heel and left ball.
And: Collect feet under body.
4: Using the toes on your left foot, allow right foot to swing into split weight position.
And: Collect feet under your body and lower.

Tips and Tricks
  • HHT: Right Heel, Left Toe, and Right Toe. Keep Feather and Three Step straight, otherwise it's very easy to get out of sync with your partner and put the follower (or leader) in an uncomfortable position.
  • Arm Swing: Just like in Feather Step, but this time with your right arm swinging forward instead of your left.

Advanced Concepts
  • Tarzan: Feather Step, left side is forward. Three step, right side if forward. Just work on keeping these two straight and using your body to help you swing along. Think of Tarzan swinging in his jungles. He's using his entire body to grab onto vines and pull himself forward. Same concept in foxtrot, just without vines hanging from above and jaguars chasing you from below.