Code p3003 rough start when it is humid, stalls when idling and check engine blinks at 30MPH

Hello, is this the Denali model or Base? Were you having these stalling issues before any work was done? The reason the check engine light is flashing at 30mph is that is indicating that the running condition of the vehicle at that time is bad enough that it can ruin the Catalytic Converter. Hence the flashing.
There are probably more misfire codes stored. But does the truck run okay when you are cruising and only rough at an idle? These are known for the intake manifolds leaking. On some you can use a spray bottle with some water and spray around the intake manifold and hear the water being sucked into the engine.
With random misfire codes, the truck should have been diagnosed properly. It sounds like a vacuum leak at the intake manifold. Or you would have cylinder specific misfire codes.
When was the last time the spark plugs, and wires were changed? All of these things should have been checked.
You can take it back if you trust them and they are going to be honest and fix what's really wrong.
But you can start with having a code scan done at your local parts store. Don't start replacing parts just because that's what people think it is.
If it's been 20,000 to 30,000 miles then you're probably due for a full tune up, spark plugs, wires, fuel filter, and air filter if they have not been done. Start with a code scan first and let us know exactly what codes you're still getting. AutoZone's computer does not tell them what to replace due to a p0300 random misfire code.

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine