  We report the results of a 2019-2021 monitoring campaign with Swift and
associated target-of-opportunity observations with XMM-Newton and NuSTAR,
examining the spectral and timing behavior of the highly variable ultraluminous
X-ray source (ULX) NGC 925 ULX-3. We find that the source exhibits a 127-128
day periodicity, with fluxes typically ranging from 1e-13 to 8e-13 ergs/s/cm2.
We do not find strong evidence for a change in period over the time that NGC
925 ULX-3 has been observed, although the source may have been in a much lower
flux state when first observed with Chandra in 2005. We do not detect
pulsations, and we place an upper limit on the pulsed fraction of ~40% in the
XMM-Newton band, consistent with some previous pulsation detections at low
energies in other ULXs. The source exhibits a typical ULX spectrum that turns
over in the NuSTAR band and can be fitted using two thermal components. These
components have a high temperature ratio that may indicate the lack of extreme
inner disk truncation by a magnetar-level magnetic field. We examine the
implications for a number of different models for superorbital periods in ULXs,
finding that a neutron star with a magnetic field of ~10^12 G may be plausible
for this source. The future detection of pulsations from this source would
allow for the further testing and constraining of such models.
