We investigate an autocorrelation function of a soluble three-dimensional system, namely, the temporal coherence function of the thermal radiation field in a cube-shaped cavity for the stochastic electrical field E. In the thermodynamic limit, C_E(t) relaxes exponentially at intermediate times, but a ''long-tail'' behaviour C_0(t)=At^-4 with A<0 is predominant for long times. In the case of finite, but not too small, C_E(t) is described by an asymptotic expansion in powers of L^-1 in terms of generalized Riemann zeta functions. Surface- and shape-effects enhance the long-tail. In the case of very small cavities, we calculate an expansion of C_E(t) in terms of exp ( -L^-1 ) and cosines. An oscillatory, but not strictly periodic, long-time behaviour is observed in this case.