input: Write a python function to find the three most frequent elements in a list.
  assert find_most_frequent([3, 7, 5, 6, 2, 1, 5, 6, 3, 9]) == [(3, 2), (5, 2), (6, 2)]
  assert find_most_frequent([1, 1, 1, 2, 2, 3]) == [(1, 3), (2, 2), (3, 1)]
  assert find_most_frequent([4, 4, 4, 4, 5, 6, 6, 7, 8]) == [(4, 4), (6, 2), (5, 1)]
output: def find_most_frequent(lst):
      from collections import Counter
      counts = Counter(lst)
      return counts.most_common(3)