How to make a 2d numpy array a 3d array?

numpy.reshape(array, array.shape + (1,))

In addition to the other answers, you can also use slicing with numpy.newaxis:

>>> from numpy import zeros, newaxis
>>> a = zeros((6, 8))
>>> a.shape
(6, 8)
>>> b = a[:, :, newaxis]
>>> b.shape
(6, 8, 1)

Or even this (which will work with an arbitrary number of dimensions):

>>> b = a[..., newaxis]
>>> b.shape
(6, 8, 1)