  It has recently been shown that superconductivity in magic-angle twisted
trilayer graphene survives to in-plane magnetic fields that are well in excess
of the Pauli limit, and much stronger than the in-plane critical magnetic
fields of magic-angle twisted bilayer graphene. The difference is surprising
because twisted bilayers and trilayers both support the magic-angle flat bands
thought to be the fountainhead of twisted graphene superconductivity. We show
here that the difference in critical magnetic fields can be traced to a
$\mathcal{C}_2 \mathcal{M}_{h}$ symmetry in trilayers that survives in-plane
magnetic fields, and also relative displacements between top and bottom layers
that are not under experimental control at present. An gate electric field
breaks the $\mathcal{C}_2 \mathcal{M}_{h}$ symmetry and therefore limits the
in-plane critical magnetic field.
