Quick updated method!
As of June 2015 it seems one can not just quit and get a win, instead having to fully finish the match. Thankfully I found a cheap method around this, and it seems to be faster than the quit out method.
By loading up Forge on Halo 3 (I suppose it could be any map, but I chose Construct as the map, and Multi-Flag as the mode), just delete the opposite teams starting points. Leave one group. They're easy to spot because they have a blue floaty aura above them, as opposed to being regular arrows that indicate regular spawns. Afterwards save your map.
When you load the map/mode up, you should get a black screen with the scoreboard saying you won (It's so fast the announcer says "Game over...capture the flag!"). Fast and easy!
Hope that helps everyone out. No one seemed to be posting about these wins for those of us that still need them and were too lazy to get them out of the way sooner.
Update: For those of you that are having trouble getting it to work, I went ahead and added the map to my file share. Just be sure to play it on Multi Flag.
Update 2: Here are the new steps (as of September 2018
) to help you get started (Credit to tknology)
1) First, load up MCC with one controller and your account. No need for second controller.
2) Press X to open the Roster.
3) Select your player card and press A.
4) Select "Find Player" and press A.
5) Type 'Fooga' in the text box that comes up and press Start. You should now be on Fooga's 'Player Details' page.
6) Select "File Share" and press A.
7) Select "Map Variants" and press A.
8) Select "Construct Win Boost" and press A. Press A a second time, select "Yes" on the prompt and press A a third time. Dismiss the notification that the download is complete.
9) Now back all the way out to MCC's main menu.
10) Choose Multiplayer > Custom Games > Halo 3.
11) For MAP, choose Construct > Win Boost
12) For Game Type, choose Capture The Flag > Multi-Flag
13) Hit A on Options (no need to change anything)
14) Hit A to Start Game.
15) Once game starts, you'll get an automatic win and it loads the Carnage Report. Hit B to back out of Carnage Report. Re-select the map (Construct > Win Boost). Press RB twice to select 'Start'. Hit A to Start Game.