Found on slickdeals forum, credit goes to danb22.
http://www.xbox.com/en-US/games/h/halo3/3for3sweepstakes/default.htm
It looks like there are three entry periods and you can enter once per period.
Sweepstakes Round 1 (Jul 26-Aug 28)
Grand Prize Winner (1)
Halo 3 Legendary Edition game
Xbox LIVE 12-Month Gold Subscription
1600 Microsoft Points
First Place Winners (2)
Halo 3 Legendary Edition game
1600 Microsoft Points
Second Place Winners (3)
Halo 3 Limited Edition game
Third Place Winners (10)
Halo 3 Standard Edition game
Sweepstakes Round 2 (Aug 30-Sep 25)
Grand Prize Winner (1)
Halo 3 Legendary Edition game
Limited Edition Halo 3 Wireless Controller (Spartan)
Limited Edition Halo 3 Wireless Headset
Xbox LIVE 12-Month Gold Subscription
1600 Microsoft Points
Halo 3 Messenger Bag
Autographed copy of Eric Nylund's Ghosts of Onyx
First Place Winners (2)
Halo 3 Legendary Edition game
Halo 3 Limited Edition Wireless Controller (Brute)
Xbox LIVE 12-Month Gold Subscription
Autographed copy of Eric Nylund's Ghosts of Onyx
Second Place Winners (3)
Halo 3 Limited Edition game
Xbox LIVE 12-Month Gold Subscription
Third Place Winners (10)
Halo 3 Standard Edition game
Sweepstakes Round 3 (Sep 27-Oct 25)
Grand Prize Winner (1)
Halo 3 Legendary Edition game
Halo 3 Special Edition Xbox 360 console
Halo 3 Limited Edition Wireless Controller (Spartan)
Halo 3 Limited Edition Wireless Headset
Halo 3 Wireless Headset
Zune™ – Halo 3 Edition
Xbox 360 Halo 3 X-shaped Ibanez Xiphos Guitar with Case
Xbox LIVE 12-Month Gold Subscription
1600 Microsoft Points
Halo 3 Messenger Bag
Halo Graphic Novel Autographed by Bungie
First Place Winners (2)
Halo 3 Legendary Edition game
Halo 3 Special Edition Xbox 360 console
Limited Edition Halo 3 Wireless Controller (Brute)
Limited Edition Halo 3 Wireless Headset
Xbox LIVE 12-Month Gold Subscription
1600 Microsoft Points
Halo 3 Messenger Bag
Halo Graphic Novel Autographed by Bungie
Second Place Winners (15)
Halo 3 Limited Edition game
Halo 3 Messenger Bag
Third Place Winners (15)
Halo 3 Standard Edition game
http://www.xbox.com/en-US/games/h/halo3/3for3sweepstakes/default.htm
It looks like there are three entry periods and you can enter once per period.
Sweepstakes Round 1 (Jul 26-Aug 28)
Grand Prize Winner (1)
Halo 3 Legendary Edition game
Xbox LIVE 12-Month Gold Subscription
1600 Microsoft Points
First Place Winners (2)
Halo 3 Legendary Edition game
1600 Microsoft Points
Second Place Winners (3)
Halo 3 Limited Edition game
Third Place Winners (10)
Halo 3 Standard Edition game
Sweepstakes Round 2 (Aug 30-Sep 25)
Grand Prize Winner (1)
Halo 3 Legendary Edition game
Limited Edition Halo 3 Wireless Controller (Spartan)
Limited Edition Halo 3 Wireless Headset
Xbox LIVE 12-Month Gold Subscription
1600 Microsoft Points
Halo 3 Messenger Bag
Autographed copy of Eric Nylund's Ghosts of Onyx
First Place Winners (2)
Halo 3 Legendary Edition game
Halo 3 Limited Edition Wireless Controller (Brute)
Xbox LIVE 12-Month Gold Subscription
Autographed copy of Eric Nylund's Ghosts of Onyx
Second Place Winners (3)
Halo 3 Limited Edition game
Xbox LIVE 12-Month Gold Subscription
Third Place Winners (10)
Halo 3 Standard Edition game
Sweepstakes Round 3 (Sep 27-Oct 25)
Grand Prize Winner (1)
Halo 3 Legendary Edition game
Halo 3 Special Edition Xbox 360 console
Halo 3 Limited Edition Wireless Controller (Spartan)
Halo 3 Limited Edition Wireless Headset
Halo 3 Wireless Headset
Zune™ – Halo 3 Edition
Xbox 360 Halo 3 X-shaped Ibanez Xiphos Guitar with Case
Xbox LIVE 12-Month Gold Subscription
1600 Microsoft Points
Halo 3 Messenger Bag
Halo Graphic Novel Autographed by Bungie
First Place Winners (2)
Halo 3 Legendary Edition game
Halo 3 Special Edition Xbox 360 console
Limited Edition Halo 3 Wireless Controller (Brute)
Limited Edition Halo 3 Wireless Headset
Xbox LIVE 12-Month Gold Subscription
1600 Microsoft Points
Halo 3 Messenger Bag
Halo Graphic Novel Autographed by Bungie
Second Place Winners (15)
Halo 3 Limited Edition game
Halo 3 Messenger Bag
Third Place Winners (15)
Halo 3 Standard Edition game