Find max positive product of three integer in an array

In careercup.com, I have seen a problem. It basically asks to get an integer array(includes positives and negatives) and return maximum positive product of three distinct element in it. Sorting is not allowed.
For example:

arr = [9, 10, -12, 4, 7, -8, -13, 4, -8, 12] 
elements = (-13, -12, 12)

There are two cases: either P1*P2*P3 or N1*N2*P1(Pi stands for ith biggest and Ni stands for ith smalltest number.). So, the algorithm finds 2 elements in the array by partitioning. One is the 2nd smallest  and the other is 3rd biggest number. Then, among biggest 2 element, it finds the maximum. The latest form of array is as follows:

N1,N2,...........,P3,P2,P1

It compares N1*N2 ith. P3*P2. It returns elements of bigger one with P1.

Expected running time of my solution is O(n). It uses randomized-select.  The python code for solution is as follows:

from random import random

def randomized_partition(arr, bottom, top):
    pv_ind = int(random()* (top-bottom)) +bottom

    pivot = arr[pv_ind]
    arr[pv_ind] = arr[top-1]
    arr[top-1] = pivot

    part_ind = bottom
    for i in xrange(bottom, top-1):
        if arr[i] < pivot:
            swap(arr, i, part_ind)
            part_ind+=1

    swap(arr, part_ind, top-1)
    return part_ind

def swap(arr, i, j):
    temp = arr[i]
    arr[i] = arr[j]
    arr[j] = temp

def randomized_select(arr, i, bottom, top):
    if not (bottom+1) < top:
        return 
    pivot = randomized_partition(arr, bottom, top)
    if i > pivot:
        randomized_select(arr, i, pivot+1, top)
    elif i < pivot:
        randomized_select(arr, i, bottom, pivot)



def find_desired_elements(arr, nthsmall, nthbig, bottom, top):
    pivot = randomized_partition(arr, bottom, top)
    if (pivot>nthsmall-1) & (pivot < nthbig+1):
        randomized_select(arr, nthsmall, bottom, pivot)
        randomized_select(arr, nthbig, pivot+1, len(arr))

    elif pivot>0:
        find_desired_elements(arr, nthsmall, nthbig, bottom, pivot)
    else:
        find_desired_elements(arr, nthsmall, nthbig, pivot+1, len(arr))


def find_max_multiplier(arr):
    find_desired_elements(arr, 1, len(arr)-3, 0, len(arr))

    if arr[len(arr)-2] > arr[len(arr)-1]:
        swap(arr, len(arr)-2, len(arr)-1)
    mult1 = arr[0] * arr[1]
    mult2 = arr[len(arr)-3] * arr[len(arr)-2]

    if mult1 == max(mult1, mult2):
        return (arr[0], arr[1], arr[len(arr)-1])
    return (arr[len(arr)-3], arr[len(arr)-2] , arr[len(arr)-1])

if __name__ == '__main__':

    arr = []
    for i in xrange(10):
        arr.append(int(random()*-100)+50)

    print arr
    result = find_max_multiplier(arr)
    print arr
    print result

P.s: Please leave a comment if you find any error. I have performed several tests on it. All has passed.