  Spiral structures are important drivers of the secular evolution of disc
galaxies, however, the origin of spiral arms and their effects on the
development of galaxies remain mysterious. In this work, we present two
three-armed spiral galaxies at z~0.3 in the Middle Age Galaxy Properties with
Integral Field Spectroscopy (MAGPI) survey. Taking advantage of the high
spatial resolution (~0.6'') of the Multi-Unit Spectroscopic Unit (MUSE), we
investigate the two-dimensional distributions of different spectral parameters:
Halpha, gas-phase metallicity, and D4000. We notice significant offsets in
Halpha (~0.2 dex) as well as gas-phase metallicities (~0.05 dex) among the
spiral arms, downstream and upstream of MAGPI1202197197 (SG1202). This
observational signature suggests the spiral structure in SG1202 is consistent
with arising from density wave theory. No azimuthal variation in Halpha or
gas-phase metallicities is observed in MAGPI1204198199 (SG1204), which can be
attributed to the tighter spiral arms in SG1204 than SG1202, coming with
stronger mixing effects in the disc. The absence of azimuthal D4000 variation
in both galaxies suggests the stars at different ages are well-mixed between
the spiral arms and distributed around the disc regions. The different
azimuthal distributions in Halpha and D4000 highlight the importance of time
scales traced by various spectral parameters when studying 2D distributions in
spiral galaxies. This work demonstrates the feasibility of constraining spiral
structures by tracing interstellar medium (ISM) and stellar population at
z~0.3, with a plan to expand the study to the full MAGPI survey.
