  We apply simulated tempering and magnetizing (STM) Monte Carlo simulations to
the two-dimensional three-state Potts model in an external magnetic field in
order to investigate the crossover scaling behaviour in the temperature-field
plane at the Potts critical point and towards the Ising universality class for
negative magnetic fields. Our data set has been generated by STM simulations of
several square lattices with sizes up to 160 x 160 spins, supplemented by
conventional canonical simulations of larger lattices at selected simulation
points. We present careful scaling and finite-size scaling analyses of the
crossover behaviour with respect to temperature, magnetic field and lattice
size.
