  Most stars form in multiple star systems. For a better understanding of their
formation processes, it is important to resolve the individual protostellar
components and the surrounding envelope and disk material at the earliest
possible formation epoch because the formation history can be lost in a few
orbital timescales. Here we present the ALMA observational results of a young
multiple protostellar system, IRAS 04239+2436, where three well-developed large
spiral arms were detected in the shocked SO emission. Along the most
conspicuous arm, the accretion streamer was also detected in the SO$_2$
emission. The observational results are complemented by numerical
magneto-hydrodynamic simulations, where those large arms only appear in
magnetically weakened clouds. The numerical simulations also suggest that the
large triple spiral arms are the result of gravitational interactions between
compact triple protostars and the turbulent infalling envelope.
