After filling it with about 3 gallons Iit wont run?

Hello, I'm Danny.

Here is the information you requested. It's possible you may have a weak or failing fuel pump causing this hard-no start condition. I would suggest performing a fuel pressure test. Here is a tutorial showing how to perform a fuel pressure test:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

The fuel pressure specification for your truck is to be 55-62 PSI.

If the fuel pressure is not within specifications you will have to drop the fuel tank to replace the in-tank fuel pump. Here is a tutorial showing how to replace an in-tank electric fuel pump:

https://www.2carpros.com/articles/how-to-replace-an-electric-fuel-pump

I've attached picture steps below on how to remove and replace both the fuel pump and fuel tank on your Chevrolet truck. Hope this helps and thanks for using 2CarPros.