import numpy as np
# create a 3-dimensional array of shape (10, 10, 10)
arr = np.arange(1000).reshape((10, 10, 10))
# find the index of the 100th element using argpartition function
indices = np.argpartition(arr.flatten(), 99)[:100]
index = np.unravel_index(indices[-1], arr.shape)
print(index)