  The merging system SIT 45 (UGC 12589) is an unusual isolated galaxy triplet,
consisting of three merging late-type galaxies, out of 315 systems in the SIT
(SDSS-based catalogue of Isolated Triplets). The main aims of this work are to
study its dynamical evolution and star formation history (SFH), as well as its
dependence on its local and large-scale environment. To study its dynamics,
parameters such as the velocity dispersion ($\sigma_{v}$), the harmonic radius
($R_{H}$), the crossing time ($H_0t_c$), and the virial mass ($M_{vir}$), along
with the compactness of the triplet ($S$) were considered. To constrain the
SFH, we used CIGALE to fit its observed spectral energy distribution using
multi-wavelength data from the ultraviolet to the infrared. According to its
SFH, SIT 45 presents star-formation, where the galaxies also present recent
($\sim $200 Myr) star-formation increase, indicating that this activity may
have been triggered by the interaction. Its dynamical configuration suggests
that the system is highly evolved in comparison to the SIT. However this is not
expected for systems composed of star-forming late-type galaxies, based on
observations in compact groups. We conclude that SIT 45 is a system of three
interacting galaxies that are evolving within the same dark matter halo, where
its compact configuration is a consequence of the on-going interaction, rather
than due to a long-term evolution (as suggested from its $H_0t_c$ value). We
consider two scenarios for the present configuration of the triplet, one where
one of the members is a tidal galaxy, and another where this galaxy arrives to
the system after the interaction. Both scenarios need further exploration. The
isolated triplet SIT 45 is therefore an ideal system to study short timescale
mechanisms ($\sim 10^8$ years), such as starbursts triggered by interactions
which are more frequent at higher redshift.
