matrix_3d

matrix_3d = [
    [
        [1, 2, 3, 4],
        [5, 6, 7, 8]
    ],
    [
        [9, 10, 11, 12],
        [13, 14, 15, 16]
    ]
]

print(matrix_3d[0])
print(matrix_3d[0][0])
print(matrix_3d[0][0][0])
print(matrix_3d[1])
print(matrix_3d[1][0])
print(matrix_3d[1][1])
print(matrix_3d[1][1][0])


Output:
[[1, 2, 3, 4], [5, 6, 7, 8]]
# [1, 2, 3, 4]
# 1
# [[9, 10, 11, 12], [13, 14, 15, 16]]
# [9, 10, 11, 12]
# [13, 14, 15, 16]
# 13