If other people (aside form yourself) will be using this class, it seems to me that returning an object would encourage some kind of uniformity in data types. If the Coord3D class has a method or property to access these coordinates as a tuple, then that still gives them that option, should they need it:
# get the object
coord_obj = my_obj.getCoordinate(my_index)
# get the tuple (for example, via a property named "coords")
coord_tup = my_obj.getCoordinate(my_index).coords
