  We report on the \nustar{} observation of the atoll type neutron star (NS)
low-mass X-ray binary GX~3+1 performed on 17 October 2017. The source was found
in a soft X-ray spectral state with $3-70$keV luminosity of
$L_\text{X}\sim3\times 10^{37}$ ergs s$^{-1}$ ($\sim 16\%$ of the Eddington
luminosity), assuming a distance of 6 kpc. A positive correlation between
intensity and hardness ratio suggests that the source was in the banana branch
during this observation. The broadband $3-70$keV \nustar{} spectral data can be
described by a two-component continuum model consisting of a disk blackbody
($kT_\text{disc}\sim1.8$keV) and a single temperature blackbody model
($kT_\text{bb}\sim2.7$keV). The spectrum shows a clear and robust indication of
relativistic reflection from the inner disc which is modelled with a
self-consistent relativistic reflection model. The accretion disc is viewed at
an inclination of $i\simeq22^\circ-26^\circ$ and extended close to the NS, down
to $R_\text{in}=(1.2-1.8) R_\text{ISCO}\:(\simeq6.1-9.1\,R_{g}\: \text{or}\:
14-20.5$ km) which allows an upper limit on the NS radius ($\leq13.5$ km).
Based on the measured flux and the mass accretion rate, the maximum radial
extension for the boundary layer is estimated to be $\sim6.3\:R_{g}$ from the
NS surface. However, if the disc is not truncated by the boundary layer but by
the magnetosphere, an estimated upper limit on the polar magnetic field would
be of $B\leq6\times10^{8}$ G.
