  Coherence, which represents the superposition of orthogonal states, is a
fundamental concept in quantum mechanics and can also be precisely defined
within quantum resource theory. Thus exploring quantum coherence in neutrino
oscillations can not only help in examining the intrinsic quantum nature but
can also explore their potential applications in quantum information
technologies. Previous studies on quantum coherence have focused on neutrino
flavor oscillations (FO). However, FO imply that neutrinos have mass and this
can lead to the generation of a tiny but finite magnetic dipole moment of
neutrinos through quantum loop diagrams at higher orders of perturbative
expansion of the interaction. This electromagnetic property of neutrinos can
induce spin flavor oscillations (SFO) in the presence of an external magnetic
field and hence is expected to enrich the study of coherence. In this work, we
investigate quantum coherence in neutrino SFO with three flavor mixing within
the interstellar as well as intergalactic magnetic fields, quantified by the
$l_1$ norm and the relative entropy of coherence, and express these measures in
terms of neutrino SFO probabilities. For FO, coherence measures can sustain
higher values (say, within 50% of the maximum) over distances of several
kilometers, which are relevant for terrestrial experiments like reactor and
accelerator neutrinos. However, for SFO, we find that the coherence scale can
extend to astrophysical distances, spanning from kiloparsecs to gigaparsecs.
