  Variability is a powerful tool to investigate properties of X-ray binaries
(XRB), in particular for Ultraluminous X-ray sources (ULXs) that are mainly
detected in the X-ray band. For most ULXs the nature of the accretor is
unknown, although a few ULXs have been confirmed to be accreting at
super-Eddington rates onto a neutron star (NS). Monitoring these sources is
particularly useful both to detect transients and to derive periodicities,
linked to orbital and super-orbital modulations. Here we present the results of
our monitoring campaign of the galaxy NGC 925, performed with the Neil Gehrels
Swift Observatory. We also include archival and literature data obtained with
Chandra, XMM-Newton and NuSTAR. We have studied spectra, light-curves and
variability properties on days to months time-scales. All the three ULXs
detected in this galaxy show flux variability. ULX-1 is one of the most
luminous ULXs known, since only 10% of the ULXs exceed a luminosity of
$\sim$5$\times$10$^{40}$ erg s$^{-1}$, but despite its high flux variability we
found only weak spectral variability. We classify it as in a hard ultraluminous
regime of super-Eddington accretion. ULX-2 and ULX-3 are less luminous but also
variable in flux and possibly also in spectral shape. We classify them as in
between the hard and the soft ultraluminous regimes. ULX-3 is a transient
source: by applying a Lomb-Scargle algorithm we derive a periodicity of $\sim$
126 d, which could be associated with an orbital or super-orbital origin.
