  We have conducted a near-infrared monitoring campaign at the UK InfraRed
Telescope (UKIRT), of the Local Group spiral galaxy M33 (Triangulum). The main
aim was to identify stars in the very final stage of their evolution, and for
which the luminosity is more directly related to the birth mass than the more
numerous less-evolved giant stars that continue to increase in luminosity. In
this third paper of the series, we measure the dust production and rates of
mass loss by the pulsating Asymptotic Giant Branch (AGB) stars and red
supergiants. To this aim, we combined our time-averaged near-IR photometry with
the multi-epoch mid-IR photometry obtained with the Spitzer Space Telescope.
The mass-loss rates are seen to increase with increasing strength of pulsation
and with increasing bolometric luminosity. Low-mass stars lose most of their
mass through stellar winds, but even super-AGB stars and red supergiants lose
$\sim40$% of their mass via a dusty stellar wind. More than three-quarters of
the dust return is oxygenous. We construct a 2-D map of the mass-return rate,
showing a radial decline but also local enhancements due to agglomerations of
massive stars. We estimate a total mass-loss rate of 0.004--0.005 M$_\odot$
yr$^{-1}$ kpc$^{-2}$, increasing to $\sim0.006$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$
when accounting for eruptive mass loss (e.g., supernov{\ae}); comparing this to
the current star formation rate of $\sim0.03$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$ we
conclude that star formation in the central region of M\,33 can only be
sustained if gas is accreted from further out in the disc or from
circum-galactic regions.
