  In the first paper of this series, we studied the effect of baryon acoustic
oscillations (BAO), redshift space distortions (RSD) and weak lensing (WL) on
measurements of angular cross-correlations in narrow redshift bins. Paper-II
presented a multitracer forecast as Figures of Merit (FoM), combining a
photometric and spectroscopic stage-IV survey. The uncertainties from galaxy
bias, the way light traces mass, is an important ingredient in the forecast.
Fixing the bias would increase our FoM equivalent to 3.3 times larger area for
the combined constraints. This paper focus on how the modelling of bias affect
these results. In the combined forecast, lensing both help and benefit from the
improved bias measurements in overlapping surveys after marginalizing over the
cosmological parameters. Adding a second lens population in counts-shear does
not have a large impact on bias error, but removing all counts-shear
information increases the bias error in a significant way. We also discuss the
relative impact of WL, magnification, RSD and BAO, and how results change as a
function of bias amplitude, photo-z error and sample density. By default we use
one bias parameter per bin (with 72 narrow bins), but we show that the results
do not change much when we use other parameterizations, with at least 3
parameters in total. Bias stochasticity, even when added as one new free
parameter per bin, only produce moderate decrease in the FoM. In general, we
find that the degradation in the figure of merit caused by the uncertainties in
the knowledge of bias is significantly smaller for overlapping surveys.
