Chain a multiple of 4 ch
Row 1: 3dc in 4th ch from hook, *skip next 3chs, [sc, ch3, 3dc] in next ch; repeat from * across to last 4chs, skip next 3chs, sc in last ch.
Row 2: ch4, turn; 3dc in base of ch, *[sc, ch3, 3dc] in next ch sp; repeat from * across to last ch sp, sc in last ch sp.
Repeat row 2 for pattern.
Happy Crocheting!