So this is probably a stupid question and common sense since I can't find the info anywhere but let's say theoretically if I swap a third a5.3 truck la motor into a third gen am I able to use the factory computer and wiring harness without having to "tune" anything like just plug in and connect everything?

 

I take it you want to use the factory 5.3 computer correct? The factory computer from your 3rd gen wont work at all with a 5.3.

You will have to slim down the 5.3 harness and remove some stuff your 3rd gen doesn't have, and there are a ton of threads dedicated to doing that. You will also have to tune the 5.3 computer to remove the VATS and a few other things not needed for your 3rd gen.

 

Welcome aboard thirdgen.org.

If you mean "Gen III 5.3", then yes, this is your basic "LS swap". There is a sticky in the top section of this forum with the basics of the swap (details depend upon the source of your 5.3). There is a link within that thread about what you need to consider when starting with a truck engine.

As jharrison5 said, the computer in your car won't work with the 5.3. In fact, you can remove it as anything it does can be handled by the 5.3 PCM. Some rewiring required.