input: Write a python function to find the three most common elements in a list using the Counter class.
  assert find_most_common([2, 5, 3, 7, 8, 5, 10, 19, 20]) == [(5, 2), (2, 1), (3, 1)]
  assert find_most_common([1, 2, 2, 3, 3, 3, 4, 4, 4, 4]) == [(4, 4), (3, 3), (2, 2)]
  assert find_most_common([10, 20, 30, 40, 50, 60, 70, 80, 90]) == [(10, 1), (20, 1), (30, 1)]
output: def find_most_common(lst):
      from collections import Counter
      counter = Counter(lst)
      return counter.most_common(3)