The shop installed three wrong fuel injectors in.

You didn't notice any symptoms while driving it? The two repairs don't correlate. If the injectors were flowing too much fuel, that would overheat the catalytic converter but the raw gas helps cool the valves. The extra gas won't burn in the cylinders if it doesn't have the air to go with it. If the injectors are too lean, that can lead to overheating and burning the valves if the condition is severe enough, but the extra unburned oxygen will be detected by the oxygen sensor, then the Engine Computer will command more fuel from all the injectors. We rarely see burned valves today because the metal alloys are much better than years ago. When some cylinders are too lean we feel that as low power, and a hesitation or stumble when accelerating.

GM has always had an issue with injectors that were not flow-matched. They can cause running and performance problems but that usually doesn't show up until there's higher mileage on those engines. Chrysler buys their injectors in flow-matched sets and has almost no trouble with them. Even with GM's problems, they don't cause valve and converter damage. With a lack of symptoms, it's hard to imagine injectors causing so much trouble. We replace them all the time for leaking way too much fuel into the engine and they often get plugged with varnish deposits that reduce the flow so much the engine has a misfire. Still, no damage to the engine. I wouldn't say for certain, but could the second shop have added unnecessary work because they saw the chance to blame someone else?

I guess I'd be more convinced if I removed the valves myself and saw that they were in the cylinders with the wrong injectors. Also, what did they mean by "wrong" injectors? There can be different part numbers that are meant for the same applications. If any improvement is made or there's some other change, they usually get a new part number to differentiate them. As long as those changes don't affect the flow rate, mixing them shouldn't matter. They could also have come from different suppliers. Sometimes a number of aftermarket companies buy their injectors from the original manufacturer, then put their own part numbers on them. The auto parts store that supplied the injectors to the first shop could have had only five left from one company, and included three from a different company. That's not how we would like to buy our parts, but that doesn't mean they can't work.

You will need to seek legal advice this is not a mechanical question and we can't advise on such matters.