Just got my Astrodon 3nm OIII filter today so I'm giving it a go tonight.
Just to see the difference under matching conditions I left the old 8.5nm OIII filter in the wheel and did a couple of images to compare.
I did a 30 minute exposure with each.
The first image shows both images with the same histogram stretch applied to both. The 3nm is on the right.
The second image shows both images with an auto screen stretch applied to both. Again the 3nm is on the right.
Interesting?
P.S. I know that 3nm image is out of focus, I've fixed that now