3 sequential numbers

Alright geniuses, if you think your programming ability is up to par, lets see you solve this:

Write an algorithm which in O(n) time finds 3 indices i, j, and k such that i < j < k and array[i] < array[j] < array[k].

You can do this in O(n) for a sorted list. But I suspect not for an unsorted list. Can't think of a proof for it at the moment, though.