  The GRIFFIN spectrometer at TRIUMF-ISAC has been used to study excited states
and transitions in $^{50}$Sc following the $\beta$-decay of $^{50}$Ca.
Branching ratios were determined from the measured $\gamma$-ray intensities,
and angular correlations of $\gamma$ rays have been used to firmly assign the
spins of excited states. The presence of an isomeric state that decays by an
$M3$ transition with a $B(M3)$ strength of 13.6(7)\,W.u. has been confirmed. We
compare with the first {\it ab initio} calculations of $B(M3$) strengths in
light and medium-mass nuclei from the valence-space in-medium similarity
renormalization group approach, using consistently derived effective
Hamiltonians and $M3$ operator. The experimental data are well reproduced for
isoscalar $M3$ transitions when using bare $g$-factors, but the strength of
isovector $M3$ transitions are found to be underestimated by an order of
magnitude.
