A week ago I bought 3 goldfish, 2 gold and one black. 2 days later one gold one died, I replaced it and then the next night the black one died, so agan I replaced it and the third one died. Today I was left with 2 one gold and one black and I have just lost the gold one. I do not understand why this keeps happening as the water has been oxygenated and purified, there is an oxygen plant and stones in the bowl. I have ownedfish in the past that have lived for over 15 years and have never had this problem before. The initial 3 fish were bought from the same pet shop and those have all died. The 4th one I bought died today and I have One black one left who seems to be doing OK, but worried we lose him too. Please help with some advise as to what I can do.

Your removing chlorine with the tablets or leaving the water out after it's drawn from the tap for 24 hours.

How about adjusting the ph to 7.0.

How about aclimating the goldfish to the new water. Lots of methods, some better than others. Can be as simple as placing the goldfish in their bag and let it sit in the water for an hour or so, to dilutions of the water the fish were brought home in.

Is this a bowl or aquarium?

Never buy goldfish in bulk, buy them one at a time and leave at least 1 week before buying another. The ammonia and co2 they release into the water can overpower the good bacteria in your filter system and the fish literally poison themselves to death. (Don't forget that a goldfish needs at least 10 gallons of water per fish to be healthy)
Here's a fantastic website to help you The Goldfish and Aquarium Board (GAB)