Unfortunately, no you cannot. Although, if you happen to make another Xbox Live account, you could start from the very beginning.
once you get to the highest level you reset
you can't reset your online stats in halo 3!
earn it.
Matchmaking
You must get to the rank Luitenant in a game type or for your rank.
rank up to brigadier and buy it for 300,000 credits.
you have to rank up to seargent
Get your overall rank to Lieutenant
I believe Lieutenant.
5 star general
it means to get a higer rank it doesnt make you better at halo 3 but what i said before it some times makes you a higher rank :)
It's call a 5 star General and it is the top rank in Halo 3