What is a neutrino state if not a particle?

When reading about the 2015 Nobel prize and how this led to the possibility of the existence of sterile neutrinos I am told that:

"(…) three active neutrinos $nu_e$, $nu_mu$, $nu_tau$, are superpositions of three massive neutrinos $nu_1 $, $nu_2 $ ,$nu_3 $ with respective masses $m_1$, $m_2$, $m_3$"(…)

I know that there are only 3 neutrinos in the standard model, but the way this is written it makes it sound like there are 6 neutrinos, and 3 of them are consequences of the interaction of the other 3. I know this must be incorrect, but I don’t know why.

I have further read about neutrino oscillations and superposition of flavour states and neutrino masses but I don’t understand this quantum superposition of mass states of neutrinos and how these do not represent the existence of extra physical particles. I understand that it works like the superposition of waves, but I don’t see how this works with particles (and I also get confused due to the particle-wave duality).