How terrible. I'm so sorry. For 3 deaths to occur so close together I would think the cause would either be...
If one had some sort of virus or virulent bacteria it could quickly spread to the others. Did you quarantine the fish before moving them into the same aquarium?
For an environmental factor to cause three deaths in a short time it would have to be something severe such as a rapid pH fluctuation, exposure to chlorine, exposure to soap, cleaning products or other chemical accidentally introduced into the aquarium or possibly heavy metal poisoning.
Do either of these possibilities seem like they could be the cause?