aguita,
What is different about your 3 acre pond compared to the other ponds besides depth.
Do all ponds have aeration?
Did the 3 acre pond freeze over completely and have snow on top of the ice.
Did the diffuser set at 4 foot depth keep a hole in the pond all winter?
Are all ponds pellet fed? If yes is the same food used for all ponds?
Do you stock HSB at the same time in the ponds? From Bruce Condello “ Don�t buy, handle, or stock HSB when water temperatures are below 50. Your fish will get a nasty green fungus on their fins and may die.”
How old, how big and how many HSB were in the 3 acre pond before the kill as compared to your other ponds?
Hopefully these questions will help you figure out what happened?