Few possibilities: (Black Molly owner myself)
1. What size is your tank?
2. Did all 3 of them come from the same tank at the LFS
3. Was the LFS
tank filled with ONLY Black Mollies?
4. If #2, was the tank separated by Male / Female? Or were they co-ed at the LFS
- Many times, if Female Black Mollies are kept together, but without contact from healthy males - they will develop a dominance among their "sorority". Usually once the male tries to mate with her enough, she'll calm down.
- OR, she may already be pregnant. Females will become naturally dominant when they're eating for 40 instead of 1. :P
- If she looks like she's causing harm to the other 2, you may need to separate her from them in another tank, to give her time to cool off.