  Through an extensive set of realistic synthetic observations (produced in
Paper I), we assess in this part of the paper series (Paper III) how the choice
of observational techniques affects the measurement of star-formation rates
(SFRs) in star-forming regions. We test the accuracy of commonly used
techniques and construct new methods to extract the SFR, so that these findings
can be applied to measure the SFR in real regions throughout the Milky Way. We
investigate diffuse infrared SFR tracers such as those using 24 {\mu}m, 70
{\mu}m and total infrared emission, which have been previously calibrated for
global galaxy scales. We set up a toy model of a galaxy and show that the
infrared emission is consistent with the intrinsic SFR using extra-galactic
calibrated laws (although the consistency does not prove their reliability).
For local scales, we show that these techniques produce completely unreliable
results for single star-forming regions, which are governed by different
characteristic timescales. We show how calibration of these techniques can be
improved for single star-forming regions by adjusting the characteristic
timescale and the scaling factor and give suggestions of new calibrations of
the diffuse star-formation tracers. We show that star-forming regions that are
dominated by high-mass stellar feedback experience a rapid drop in infrared
emission once high-mass stellar feedback is turned on, which implies different
characteristic timescales. Moreover, we explore the measured SFRs calculated
directly from the observed young stellar population. We find that the measured
point sources follow the evolutionary pace of star formation more directly than
diffuse star-formation tracers.
