hi there fellow modder i think...
i dont know how to do read mes but ill keep it simple.
first of all you need the latest version of assembly for halo 3 mcc modding
the file is a asmp, which is a patch.
BACK UP YOUR ORIGINAL HALO MAPS.
ok ok ok.
now that you have backed up your maps
open assembly
go 2 tools then map patcher
then what u can do is apply the asmp to the file located in your halo map folder
once you applied it the sand box original map, then you should be ready.
open halo mcc without easy anti cheat then have fun i guess.

dont dm me for help. a tutorial should come out soon. thank you.