  In a MC study using a cluster update algorithm we investigate the finite-size
scaling (FSS) of the correlation lengths of several representatives of the
class of three-dimensional classical O(n) symmetric spin models on a column
geometry. For all considered models we find strong evidence for a linear
relation between FSS amplitudes and scaling dimensions when applying
antiperiodic instead of periodic boundary conditions across the torus. The
considered type of scaling relation can be proven analytically for systems on
two-dimensional strips with periodic bc using conformal field theory
