Why did my fish die in 3 hours? (Fish bought from walmart)

Regardless of the reason that your fish actually ended up dying, I would strongly suggest that you buy your fish elsewhere in the future.  We have some chain as well as some independent pet stores in my area that are slightly more expensive when it comes to buying fish.  However, overall they take much better care of their fish and you're much more likely to get a fish that is healthier from the start.  I am fortunate because in our area we even have an entire store that is completely dedicated to the fish and pond aquarium enthusiast!  It is a really nice place to go for ideas, advice about fish, and to see all the cool and interesting types of fish you can get that you would never find at some place like Wal-Mart.