My 1st problem solution:

const solution = str => str.split('')
  .reduce((prev, curr) => {
    let lastLetter = prev.slice(-1)[0]
    if (lastLetter && lastLetter.letter === curr) {
      return [
        ...prev.slice(0, -1),
        {
          ...lastLetter,
          count: lastLetter.count + 1,
        },
      ]
    }

    return [
      ...prev,
      { letter: curr, count: 1 },
    ]
  }, [])
  .map(val => val.letter.repeat(val.count > 2 ? 2 : val.count))
  .join('')

2nd problem solution:

const solution = arr => {
  const numbersMap = arr.reduce((prev, curr) => {
    const sum = sumDigits(curr)
    if (prev[sum]) {
      prev[sum].push(curr)
      return prev
    }

    return {
      ...prev,
      [sum]: [curr],
    }
  }, {})

  let sums = []
  Object.values(numbersMap)
    .map(numbers => {
      if (numbers.length === 2) {
        sums.push(sumNumbers(numbers))
      }
      if (numbers.length > 2) { // find two greatest
        const greatest = numbers.sort()
          .slice(-2)
        sums.push(sumNumbers(greatest))
      }
    })

  if (sums.length === 1) {
    return sums[0]
  }

  if (sums.length > 1) {
    return Math.max(...sums)
  }

  return -1
}

3rd problem solution — coming soon