  We report the results of an observation of low mass X-ray binary GX 3+1 with
{\it AstroSat}'s Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray
Telescope (SXT) instruments on-board for the first time. We have detected one
Type-1 thermonuclear burst ($\sim$ 15 s) present in the LAXPC 20 light curve,
with a double peak feature at higher energies and our study of the
hardness-intensity diagram reveals that the source was in a soft banana state.
The pre-burst emission could be described well by a thermally Comptonised model
component. The burst spectra is modelled adopting a time-resolved spectroscopic
method using a single color blackbody model added to the pre-burst model, to
monitor the parametric changes as the burst decays. Based on our time-resolved
spectroscopy, we claim that the detected burst is a photospheric radius
expansion (PRE) burst. During the PRE phase, the blackbody flux is found to be
approximately constant at an averaged value $\sim$ 2.56 in $10^{-8}$ ergs
s$^{-1}$ cm$^{-2}$ units. On the basis of literature survey, we infer that
\textit{AstroSat}/LAXPC 20 has detected a burst from GX 3+1 after more than a
decade which is also a PRE one. Utilising the burst parameters obtained, we
provide a new estimation to the source distance, which is $\sim$ 9.3 $\pm$ 0.4
kpc, calculated for an isotropic burst emission. Finally, we discuss and
compare our findings with the published literature reports.
