  We present near-simultaneous Chandra/HST observations of the very faint
($L_{x} < 10^{36}$ erg s$^{-1}$) X-ray transient source M15 X-3, as well as
unpublished archival Chandra observations of M15 X-3. The Chandra observations
constrain the luminosity of M15 X-3 to be $< 10^{34}$ erg s$^{-1}$ in all
observed epochs. The X-ray spectrum shows evidence of curvature, and prefers a
fit to a broken power-law with break energy $E_{\rm break} = 2.7^{+0.4}_{-0.6}$
keV, and power law indices of $\Gamma_{1} = 1.3^{+0.1}_{-0.2}$ and $\Gamma_{2}
= 1.9^{+0.2}_{-0.2}$ over a single power law. We fit our new F438W ($B$), F606W
(broad $V$), and F814W ($I$) HST data on the blue optical counterpart with a
model for an accretion disk and a metal-poor main sequence star. From this fit,
we determine the companion to be consistent with a main sequence star of mass
$0.440^{+0.035}_{-0.060}$ $M_{\odot}$ in a $\sim$4-hour orbit. X-ray
irradiation of the companion is likely to be a factor in the optical emission
from the system, which permits the companion to be smaller than calculated
above, but larger than $0.15$ $M_{\odot}$ at the $3\sigma$ confidence level.
M15 X-3 seems to be inconsistent with all suggested hypotheses explaining very
faint transient behavior, except for magnetospherically inhibited accretion.
