DHP = Digital Horsepower. A tuning software like HPT where you don't have to purchase credits for each vehicle.
HPT= HPTuners, in my opinion its the best solution right now for 3800s becuase the company is still in business and there's other peoples tunes and a forum on their site.
Diablotuner does not work with a 3800, its a handheld tuner commonly used on trucks. It stores files and you can switch them quickly and easily.

Tuning isn't that bad if you're willing to invest the time to learn. Its definitely worth it in my opinion, not only because tuning yourself means you can tune with every change to the vehicle, but the cost savings vs a shop tuning is great as well.

You can find used hptuners for a couple hundred. It only takes 2 credits ($50 each if you have to buy them from hpt) to unlock a vehicle, then you never have to pay another dime to tune.