You will need a voltage regulator for the separate cirquit, so the battery doesn't get overcharged (unless it's already integrated into the alternator).
I have never seen a 3-wire alternator that is not internally regulated.
If this unit indeed came off of a 1987 deville:
It is a 120amp unit and it is externally regulated.
Here are some photos of things to look for:
Rear-photo of alternator
photo of external regulator
Your regulator may already be on your alternator. Any chance you can upload a photo of your alternator?? I mostly need the rear of the unit.
Also, do you already have the 3-wire pigtail? Keep in mind the photos I am showing you are from an autoparts store's online information-- so I need you to verify that it is correct.
I built a battery isolator and installed a 2nd battery in my shoebox scion for $10... it works perfectly... and is easy to do.
How about just upgrading your current alternator?
Get a high output alternator and replace your stock one.
Then rip out that wire you did for the big 3 and use 0awg.
Then get an Optima battery to replace your current one.
That should help out quite a bit. If you STILL get dimming, then think about maybe a second alt. I had around 3k watts worth of amp in my old car and it did not dim using what I just described above.
I had a 200 amp alternator.