def ndvire3(b7, b8):
    """
    Normalized Difference Vegetation Index Red-edge 3 \
    (Gitelson and Merzlyak, 1994).

    .. math:: NDVIRE3 = (b8 - b7) / (b8 + b7)

    :param b7: Red-edge 3.
    :type b7: numpy.ndarray or float
    :param b8: NIR.
    :type b8: numpy.ndarray or float

    :returns NDVIRE3: Index value

    .. Tip::

       Gitelson, A., Merzlyak, M. N. 1994. Spectral reflectance changes \
       associated with autumn senescence of Aesculus hippocastanum L. and \
       Acer platanoides L. leaves. Spectral features and relation to \
       chlorophyll estimation. Journal of Plant Physiology 143(3), 286-292. \
       doi:10.1016/S0176-1617(11)81633-0.
    """

    NDVIRE3 = (b8 - b7) / (b8 + b7)
    return NDVIRE3