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

There are a lot of factors that go into why fish die so quick in a new tank with water. It is never a simple answer and it surely isn't as simple as placing fish in water. There are water chemistry, and aeration that goes along with it.

You need to check the PH levels of the water, as well as temperature and proper aeration. Buy a testing kit and check to see what is the optimum levels for gold fish to thrive.

Also you need to acclimate the fish to the new water temperature by floating them in the bag and leave them in the tank water until the temperature of the bag and tank are the same. The fish will go in shock if you just dump them in the water and die soon after. HINT HINT.

Finally, you need to make sure that there is more then enough aeration in the tank. Let the filter and the air pump (if that is your current set up) run for a few hours or even days before the fish arrive. let the water cycle through the new mechanism and you can work out the bugs if they present themselves.