Base rnd, each side: Multiple of 6 + 3 + corner sts.
Begin 2 sts to left of corner st.
Rnd 1: Ch 1, sc in same st, *[ch 1, skip 2 sts, [(dc, ch 1, dc, ch 1, dc) in next st — open shell made, ch 1, skip 2 sts, sc in next st] to 1 st before corner st, ch 1, skip 1 st, open shell in corner st, ch 1, skip 1 st, sc in next st; rep from * around, omitting last sc, join with slip st to first sc.
Rnd 2: Ch 6 (counts as dc and ch-3), *[sc in center dc of next open shell, ch 3, dc in next sc, ch 3] to corner shell, (sc, ch 3) two times in center dc of corner shell**, dc in next sc, ch 3; rep from * around, ending last rep at **, join with slip st to 3rd ch of ch-6.
Rnd 3: Ch 4 (counts as dc and ch-1), (dc, ch 1) two times in same st, sc in next sc, ch 1, *[open shell in next dc, ch 1, sc in next sc, ch 1] to corner ch-3 space, open shell in corner space, ch 1, sc in next sc, ch 1; rep from * around, join with slip st to 3rd ch of ch-4. Fasten off.