Well I made one for Halo 3 too. It allows you to edit your campaign progress, set the difficulty at which you beat the missions, unlock skulls, change your armor, change your emblem, and unlock achievements.
If you find any bugs, please post them.
Download
Download Source
Virus Scan
How to Use this Tool:
does the armor actually work?
Can you get the flameing Helmet from this?
No, if try to set it just change it to Mark VI.
For ones you have unlocked, but you can unlock all of the armor with the achievements and skulls, besides recon and the bungie chest.
Someone should really do a rank GPD. If it’s possible.
There was already one of these but nice job 
That sucks. Anyways Looks like a great program.
I’m about to try it…
Hopefully this one works!
This keeps coming up… -___-
Idk, I posted a virus scan.
This is pretty cool, thanks for making these.
Nice!
it’s not. rank’s server sided, as opposed to games like crysis 2 and halo wars.
Thanks, now my legacy for Halo Reach will have Legendary complete on both ODST and Halo 3. My banned account and a legit legendary legacy.
EDIT:
Can you make it so it can unlock all of the skulls? When you just mod the achievements, a lot of times you do not get the hayabusa.
Why doesn’t it show when you click your profile, like if you beat it on legendary? Mine shows I’m still in progress, even though it shows I completed Co-op and Solo all in Legendary.
Idk, for me it said I completed it.
The first modding program I made edited the Halo 3 GPD as well. This one is way better. 
Please add feature to unlock skulls as i dont have campaign disc anymore
Can you get ban for this? Well, the achievement unlocking?
Any form of modding comes with a risk.
I know. I’m wondering though does the unlocking achievements part ban-free? I’ve moded my gamerscore to like 65k and I have not received a reset yet.
