Question

How does heavy water detect all 3 flavors of neutrinos?

Answer 1

The neutrino's from the sun are all of one type. The heavy water was to detect that type. The mixing with the other neutrino types resulted in 2/3's of the neutrinos not being the 'right' type for detection. –

Answer 2

The distinctive characteristic of the heavy water observatory is that it can measure both the electron neutrino flux and the total neutrino flux (electron, muon and tau neutrinos).