'**SUMMARY**
Stack three signle channel filters of the same size to create
a 3 channel filter.
**PARAMETERS**
* *flt1* - second filter to be stacked
* *flt2* - thrid filter to be stacked
**RETURNS**
DFT filter
**EXAMPLE**
>>> flt1 = DFT.createGaussianFilter(dia=200, size=(380, 240))
>>> flt2 = DFT.createGaussianFilter(dia=100, size=(380, 240))
>>> flt2 = DFT.createGaussianFilter(dia=70, size=(380, 240))
>>> flt = flt1.stackFilters(flt2, flt3) # 3 channel filter'

def stackFilters(self, flt1, flt2):

    if (not ((self.channels == 1) and (flt1.channels == 1) and (flt2.channels == 1))):
        warnings.warn('Filters    must    have    only    1    channel')
        return None
    if (not ((self.size() == flt1.size()) and (self.size() == flt2.size()))):
        warnings.warn('All    the    filters    must    be    of    same    size')
        return None
    numpyflt = self._numpy
    numpyflt1 = flt1._numpy
    numpyflt2 = flt2._numpy
    flt = np.dstack((numpyflt, numpyflt1, numpyflt2))
    img = Image(flt)
    stackedfilter = DFT(size=self.size(), numpyarray=flt, image=img, channels=3)
    return stackedfilter
