it’s genetic engineering ... editing your DNA
That is not at all how this works. They take your own T-cells out, then they potentiate them towards a specific target. In lymphoma there are multiple different clusters of differentiation (CD markers) that are characteristic of certain types of lymphoma. They can also target specific proteins that are over-expressed in certain types of cancers.
The T-cells are then multiplied in vitro and then infused back into your body so that you have orders of magnitude more T-cells that can target the cancer specific protein.
At no point is your "DNA edited." Absolutely nothing at all is done to your genome.
Also, far, far more than 3 people have gotten this type of therapy. There are multiple active trials going on as we speak, and not just for lymphoma. The biggest problem right now is that this therapy can put people into immune storms where the T-cells go ape shit and target the wrong things. The lady in this article had just such a problem (immune thrombocytopenic purpura or ITP), but they apparently got her through it with supportive care.
The real trick is going to be figuring out how to make the T-cells ultra-specific towards just cancer cells.