What is the easiest way to change the three spark plugs in the back of the engine?

Can you upload a picture of what you are talking about? The engine in these trucks sit front to back so I am not sure what's going on, but I will be glad to help you. Here is a generic guide to help you get started and some specs and parts that you will need to do the job:

Spark Plug Type .................... AWSF-32F
Gap .................... 1.32 - 1.42 mm (0.052 - 0.056 in)
Torque .................... 15 Nm (11 lb-ft)

https://www.2carpros.com/articles/how-to-change-spark-plugs

Check out the diagrams (below). Let us know what happens and please upload pictures or videos of the problem.