Sorry for your loss, but the person at the petstore misinformed you. A group of three is a deadly combination, as the two higher ranking girls will often kill the lowest, and then proceed to try and kill each other. Four is sometimes okay, but five is definitely better. Sororities are very stressful for the fish, so they often end due to deadly stress-related illnesses or a large fight. My tank ended up getting columnaris, which killed off about ten of my girls. I have five left now in my 36 gallon tank. Others have been picked off for being weak as well, due to the inherent nature of the species. You can try to babysit them all day, but its always when you aren't looking that something bad happens.
I've learned my lesson, and as much as I love the look of sororities, I won't ever try one again. Once these girls are gone (which hopefully won't be soon, as I love my girls to death) I am sticking to peaceful community fish.