Video showing the issue: https://www.youtube.com/watch?v=KMZcd_NLzeM
Halo 3 has some big diagonal acceleration zones, which makes the aiming go faster than you want when you only want to move gently to those directions. This makes the Halo MCC an inconsistent gameplay experience when playing all the Halos with the same Modern Aiming settings. Depending of the tittle you play you get better or worse aiming and accuracy between Halos, and that only makes sense if you are playing with the Classic aiming because they originally had different sensitivity curves and acceleration zones, but when selecting Modern Aiming in all Halos they should all feel and play the same when it comes about sensitivity and acceleration (I am not talkng about Aim Assist or Bullet Magnetism, I perfectly know that those are very different things and they are also different in every Halo which is fine).
Really? I’m playing it right now and it feels the same.
Yep, there is a difference. I would love to have some debug and modding tools to show it with numbers but for now I only have that video to show it.
You should submit a ticket with 343 then.
I did, one year ago 
Submit another. Try to provide as much info as possible and don’t let them close it out.
I’ve had a few where some got closed out due to inactivity.
Was playing a cross campaign playlist today, Everything was great till I got to Halo 3 and found the right stick sensitivity was way too high. Altering the settings in options did nothing. Everything seems to work fine in the other games but 3 is completely unplayable right now.
If there is a way to submit a trouble ticket to have this addressed, could someone please point me in that direction?
Yes. Here: https://support.halowaypoint.com/hc/en-us