group two elements each `n` elements with a nice offset :)

groupArrayEach.js

const transformIndex = (index, groupEach, offset) => 
  offset + (index - offset - Math.ceil((index - offset) / groupEach))
const transformIndexEachThreeWithOneOffset = (index) => transformIndex(index, 3, 1)

function groupArrayEach(arr, groupEach=3, offset=0) {
  return arr.reduce(function(acc, e, index) {
    const newIndex = transformIndex(index, groupEach, offset)
    acc[newIndex] = acc[newIndex] || []
    acc[newIndex].push(e)
    return acc
  }, [])
}


console.log(group(Array(18).fill('').map((e, i)=>i)))

Array(20).fill('').map((e, i)=>i).map(transformIndexEachThreeWithOneOffset)