You need to create the copy of the object. You may do it using numpy.copy() since you are having numpy object. Hence, your initialisation should be like:

imageEdited_3d = imageOriginal_3d.copy()

Also there is copy module for creating the deep copy OR, shallow copy. This works independent of object type. For example, your code using copy should be as:

from copy import copy, deepcopy

# Creates shallow copy of object
imageEdited_3d = copy(imageOriginal_3d)

# Creates deep copy of object
imageEdited_3d = deepcopy(imageOriginal_3d)

Description:

A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.

A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.