I've heard from dmm players that the chances are pretty low overall. JP Wiki says they can drop from rank3 acc quests too, but I'm assuming it's probably even rarer there. Though one of your union mates did get a SSR brooch from tier 3 today.
Very jealous. Not sure if I'll even have enough runes to get enough for my light girls if the drop rate is really low. I might go and farm rank3 instead since it's just that less stressful.