  Nuclei are rather classical systems in a sense. In the old days, their
phenomena were roughly explained in classical rules such as the liquid drop
model. This fact may be understood that when we see an finite quantum many body
system like nucleus, though which is a group of quantum mechanical particles,
its any collective degree of freedom has any classicality. Getting a
classicality does not depend on spatial scales of objects. It is made by a
phenomenon called Quantum Decoherence. We have studied about Quantum
Decoherence in a finite system as nucleus. In this paper, at the harmonic three
body problem (3 Schroedinger cats), it is shown that one degree of freedom as a
sub-system would get classicality because of the other two degrees of freedom.
Therefore we can assume that a nuclear collective degree of freedom would get
classicality when it couples with any other internal degrees of freedom. In
this paper, we also note that there is some relationship between the Quantum
Decoherence and Crossing of classical orbits in 3 Schroedinger cats model.
