  We study the probability distribution P(M) of the order parameter (average
magnetization) M, for the finite-size systems at the critical point. The
systems under consideration are the 3-dimensional Ising model on a simple cubic
lattice, and its 3-state generalization known to have remarkably small
corrections to scaling. Both models are studied in a cubic box with periodic
boundary conditions. The model with reduced corrections to scaling makes it
possible to determine P(M) with unprecedented precision. We also obtain a
simple, but remarkably accurate approximate formula describing the universal
shape of P(M).
