Cast on a multiple of 4 + 2 stitches
1st and 3rd rows: Purl
2nd row: K1, *(K1, P1, K1) into the next st, P3tog, repeat from * to last st, k1
4th row: K1, *P3tog, (K1, P1, K1) into the next st, repeat from * to last stitch, k1
Rows 1 - 4 form pattern. Repeat for desired length.