Stress is the number one cause of Betta Death. It lowers their immune systems, and then bacteria sneaks in and attacks!
I would be willing to bet you got all three of the fish at the same store.
Some bettas are shipped from Thailand clear across the thousands of miles to your fish store. I would call around and find out if you have any local breeders. Any fish bred in your area will not have nearly the levels of stress that imported fish will.
(A little trick I learned a while back!)
Also, fish that are bred by a local, smaller scale breeder generally grow up accustomed to each other and their families, and make better adjusted fish.
Sadly, the average life-span for betta fish is only 6 months to a year, because of the cruel way they are treated, shipped, mis-diagnosed and cared for.
This can be increased to as much as 5 years with the use of an exhaustive Betta Care Manual.
Hope this helps.