  We focus on the transition from quantum to classical behavior in
thermodynamic functions and time correlation functions of a system consisting
of three identical quantum spins s that interact via isotropic Heisenberg
exchange. The partition function and the zero-field magnetic susceptibility are
readily shown to adopt their classical forms with increasing s. The behavior of
the spin autocorrelation function (ACF) is more subtle. Unlike the classical
Heisenberg trimer where the ACF approaches a unique non-zero limit for long
times, for the quantum trimer the ACF is periodic in time. We present exact
values of the time average over one period of the quantum trimer for s less or
equal 7 and for infinite temperature. These averages differ from the long-time
limit, (9/40)\ln3+(7/30), of the corresponding classical trimer by terms of
order 1/(s*s). However, upon applying the Levin u-sequence acceleration method
to our quantum results we can reproduce the classical value to six significant
figures.
