Harry Lime
Practitioner
Here are examples of all three films, followed by my own opinion of them
I agree with your findings.
Delta3200 is the fastest of the bunch. It is a low contrast film, which really works to your advantage when shooting in the dark. Grain is pretty good and not unattractive. Ilford claims it's true speed is 1000asa. Ilford DD-X is supposed to be the perfect developer for this film and that is what I used in the past, but I have been getting really good results with it at 1600 in Diafine. Don't freak out when you see your first negs. D3200 negs tend to look thin, as if you had made a mistake processing. But once you put them in the enlarger or scanner, all will be well...
P3200 is the second fastest, clocking in around 800asa. It doesn't have as much range as Delta3200. The whites clip and darks crush a lot faster. Also that grain.... it's beautiful, but huge. Use the Tmax developer. It gives a noticeable push to the shadows.
Neopan1600 is the slowest and clocks in at about 640asa. In my experience the shadows block up very quickly even at 1250 or 1600. But the grain is very fine. In the past I've shot it at 800 on flat overcast days. That works well and then it's a nice film.
Tri-X in Diafine at 1250 is really beautiful. Smooth, with nice highlights. The shadows aren't as good as Delta3200, but then again Tri-X is only 400 asa. But regardless, this is a real winning combination.