The Gardner was pulled with a double-helix bulletworm.
I've dug a group of unfired Minies which have a nearly-identical flattened area on one side. So, in my humble opinion, your 3-groove Minie may have been in the lowest layer of bullets in a crate of ammo that got dropped. The great weight of the layers of Minies above it crushed it against the crate's flat bottom, with a slight sliding movement forward at the moment of impact, due to the way the crate landed.
Regards,
Pete