  We report the discovery of a third ULX in NGC 925 (ULX-3), detected in
November 2017 by Chandra at a luminosity of $L_{\rm X} =
(7.8\pm0.8)\times10^{39}$ erg s$^{-1}$. Examination of archival data for NGC
925 reveals that ULX-3 was detected by Swift at a similarly high luminosity in
2011, as well as by XMM-Newton in January 2017 at a much lower luminosity of
$L_{\rm X} = (3.8\pm0.5)\times10^{38}$ erg s$^{-1}$. With an additional Chandra
non-detection in 2005, this object demonstrates a high dynamic range of flux of
factor >26. In its high-luminosity detections, ULX-3 exhibits a hard power-law
spectrum with $\Gamma=1.6\pm0.1$, whereas the XMM-Newton detection is slightly
softer, with $\Gamma=1.8^{+0.2}_{-0.1}$ and also well-fitted with a broadened
disc model. The long-term light curve is sparsely covered and could be
consistent either with the propeller effect or with a large-amplitude
superorbital period, both of which are seen in ULXs, in particular those with
neutron star accretors. Further systematic monitoring of ULX-3 will allow us to
determine the mechanism by which ULX-3 undergoes its extreme variability and to
better understand the accretion processes of ULXs.
