Solve in C++

The array a(1..n) contains arbitrary integers. Write a function reduce (a, n) that reduces the array a(1..n) by eliminating from it all values that are equal to three largest different integers. For example, if a = (9, 1, 1, 6, 7, 1, 2, 3, 3, 5, 6, 6, 6, 6, 7, 9) then three largest different integers are 6, 7, 9 and after reduction the reduced array will be a=(1, 1, 1, 2, 3, 3, 5), n=7. The solution should have time complexity O(n)