IHMO the safest way to deal with 3 batteries, is to have one male XT90 (yes, only one) on a short “pigtail” and each battery has a female XT90s on it, and you can physically only have one plugged in at a time.
This would be instead of a loopkey. These would serve as the loopkey, but also make sure that only one battery could be connected at a time,
genders: