Find the element that appears once others appears thrice

Given an array where every element occurs three times, except one element which occurs only once. Find the element that occurs once.
Expected time complexity is O(n) and O(1) extra space.
Examples:
Input: arr[] = {12, 1, 12, 3, 12, 1, 1, 2, 3, 3}
Output: 2

Find two non repeating elements in an array of repeating elements

Given an array in which all numbers except two are repeated once.
(i.e. we have 2n+2 numbers and n numbers are occurring twice and remaining two have occurred once).
Find those two numbers in the most efficient way.
example :
Arr = [1,2,3,1] => 2, 3 occurring once
Arr = [2, 3, 7, 9, 11, 2, 3, 11] => 7, 9 occurring once