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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _separable_approx3(h, N=1):
 """ returns the N first approximations to the 3d function h """

    return np.cumsum([np.einsum("i,j,k", fz, fy, fx) for fz, fy, fx in _separable_series3(h, N)], 0)