Holley is basically a start of the art ECU. It's very configurable and very expandable.
The main wiring harness can be used with any ECU but some ECU's have more harnesses than others.

If you need transmission controls for an automatic you will need to use either the Terminator X Max or the Dominator. Otherwise you can run any ECU you want

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A full breakdown of all of the capabilities is listed at the bottom of this page.

If for whatever reason you don't have enough inputs or outputs you can always expand your ECU using a CAN module.
You can also upgrade your ECU without having to buy a new wiring harness; each shares the same main harness connector and pinout for it.

The Terminator X comes with a 3.5“ LCD touch screen and the wiring harness if you buy the kit. You're almost paying for the harnesses and LCD and receiving a free ECU with it. Then if you want to upgrade down the road you could always buy a different ECU or add a CAN module.

Go ahead and download the software and play around with it to see if it can do what you're looking for. Terminator and HP/Dominator have their own software

Terminator software:
https://www.holley.com/products/fuel_systems/fuel_injection/terminator_x/terminator_x_software/parts/TERMINATORXSOFTWARE

Dominator/HP EFI software:
https://www.holley.com/support/fuel_injection/hp_and_dominator_efi/

Holley has no harnesses built into it for a 3800 as a kit, but can be configured to work with a 3800 and will just need some wiring harness modifications.
The simplest route seems to be to use the LS1 harness and modify it for the 3800. This allows you to use the LS style coil pack and retire the waste spark ignition system on the 3800.

Here is a breakdown of the sensors and harnesses that you might use:

The 3800 was equipped with different sensors depending on what car the engine went into. Because of this you may want to make sure you have the right sensors that will make the Holley happy.
The coolant temp and oil pressure sensors in particular are the ones to look out for. In general you want to look for sensors with the round 3 pin connector since these are actually sensors that output a voltage based on what they're seeing rather than a sensor that only changes the resistance across the terminals.

The recommended sensors I've found so far are:

The MAP sensor may also need to be examined to make sure you have the right terminal for the sensor you're using. I would just go recommend going with a Holley 3 bar map.

I would avoid the GM 3 bar MAP that snaps into the L36 manifold though. That MAP sensor was made for Duramax engines and doesn't have the resolution other sensors have. Would not rely on that for a car tuned in speed density.

I created a base tune as a starting point for a turbo 3800. You should go through it and make sure your sensors and injector are all correct though.
Base Tune Download

This forum thread contains a lot of pretty good information about how to make the Holley HP EFI and Dominator run on a 3800:
https://forums.holley.com/showthread.php?16249-HP-EFI-On-3800-V6-Engine/page4

One of the biggest issues in getting started that was noted there was how to send the ECU a crank signal.
The solution presented on that page was as follows:

So in review, the GM Series II & Series III 3800 engines will run just fine on the Holley system, with LS coils and using the factory crank & cam sensors.
You will need to set the Firing Order to 5,4,3,2,1,6
Use the Custom Ignition Type:
Crank - 1 Pulse/Fire - Digital Rising
Ignition Reference Angle 75° and using the 3x reluctor on balancer.
Cam - Single Pulse - Digital Falling
Inductive Delay - 100

A later post on that page mentioned it would also be a good idea to synchronize timing in Holley as well.

https://forums.holley.com/showthread.php?2577-Synchronizing-Ignition-Timing-Holley-EFI

Pulled from the bottom of this page: https://www.holley.com/products/fuel_systems/fuel_injection/terminator_x/