Sorting a list with 3 unique numbers

Hi, here's your problem today. This problem was recently asked by Google:

Given a list of numbers with only 3 unique numbers (1, 2, 3), sort the list in O(n) time.

Example 1:
Input: [3, 3, 2, 1, 3, 2, 1]
Output: [1, 1, 2, 2, 3, 3, 3]

def sortNums(nums):

  # Fill this in.



print sortNums([3, 3, 2, 1, 3, 2, 1])

# [1, 1, 2, 2, 3, 3, 3]

Challenge: Try sorting the list using constant space.