Passing the first stitch of the k3 over the 2nd and 3rd is similar to a bind off. Instead of passing the first st over the 2nd as in the bind off, pass the first stitch over the 2nd and 3rd.
The "Insert right hand needle from behind under running thread between the st just worked and next st, then purl this thread" is similar to the make one purl in this video
And yes, it looks like rows 1 and 4 are main color and rows 2 and 3 worked with A. Usually the pattern directions before the pattern stitches are defined, give you a bit more information about the use of different colors.
You could try a small swatch in the pattern rows and with the colors of yarn you'll be using just to make sure you're getting the result you want.