  Spatially resolved studies of star forming regions show that the assumption
of spherical geometry is not realistic in most cases, with a major complication
posed by the gas being ionised by multiple non-centrally located stars or star
clusters. We try to isolate the effects of multiple non-centrally located stars
on the temperature and ionisation structure of HII regions, via the
construction of 3D photoionisation models using the 3D Monte Carlo
photoionisation code MOCASSIN. We find that the true temperature fluctuations
due to the stellar distribution (as opposed to the large-scale temperature
gradients due to other gas properties) are small in all cases and not a
significant cause of error in metallicity studies. Strong emission lines from
HII regions are often used to study the metallicity of star-forming regions. We
compare integrated emission line spectra from our models and quantify any
systematic errors caused by the simplifying assumption of a single, central
location for all ionising sources. We find that the dependence of the
metallicity indicators on the ionisation parameter causes a clear bias, due to
the fact that models with a fully distributed configuration of stars always
display lower ionisation parameters than their fully concentrated counterparts.
The errors found imply that the geometrical distribution of ionisation sources
may partly account for the large scatter in metallicities derived using
model-calibrated empirical methods.
