The third kind of Rib stitches is made by combination of 2 k, 1 P and 1 k, 2 P in return row. The front side and back side is illustrated in the picture. Same as all other rib stitches the number of stitches has a main role in the first stitch being Knit or Purled. To make it straightforward it is better to cast on any multiple of three stitches.
Cast on multiple of three stitches
Row 1) Repeat ( 2K, 1P)
Row 2) Repeat (1K, 2P)
Repeat these steps
Grid below will give you a good view of what is all about.