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 } } \sim 3 \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 } } \sim 1.8 { ~ { } keV } { } ) and a single temperature blackbody model ( kT _ { \text { bb } } \sim 2.7 { ~ { } 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 \simeq 22 \degr - 26 \degr and extended close to the NS , down to R _ { \text { in } } = ( 1.2 - 1.8 ) R _ { \text { ISCO } } \ > ( \simeq 6.1 - 9.1 R _ { g } \ > \text { or } \ > 14 - % 20.5 km ) which allows an upper limit on the NS radius ( \leq 13.5 km ) . Based on the measured flux and the mass accretion rate , the maximum radial extension for the boundary layer is estimated to be \sim 6.3 \ > 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 \leq 6 \times 10 ^ { 8 } G .