I have accidentally killed three betta fish in about three weeks. What am I doing wrong?

I read about betta fish before getting one, and I am doing everything that everyone says to do. My first fish jumped out of his bowl and died, but I have since started putting mesh on the top of the bowl. I got the first two from petsmart, but after the second one died, I got one from a tropical fish specialty store. I change the water every 2-3 days, I have a heater, i feed the fish a pebble or two a day, I put betta conditioner in the water, and I monitor the temperature. What am I doing wrong?