Вы можете использовать np.dstack :
import numpy as np
image = np.random.randint(100, size=(100, 100, 3))
r, g, b = image[:, :, 0], image[:, :, 1], image[:, :, 2]
result = np.dstack((r, g, b))
print("image shape", image.shape)
print("result shape", result.shape)
Выход
image shape (100, 100, 3)
result shape (100, 100, 3)