You're never really able to find games in "Team Mythic", "Social Skirmish" and "Team Snipers". The first two never have more than 60 people at one time so that's only 7 lobbies going at one time - maximum and some will be boosters and others incompatible
- do the maths. All others just take time like all playlists, Halo 3 is a shadow of its former self.
You can't transfer DLC from one account to another, but if you bought the dlc on that console or bought it on a different console but transferred the licenses, then you should be able to use the dlc on any account/gamertag on that console. So on a new account
you should still be able to use the Halo map packs that are on your old account.
So as I said there is no point anyway and you simply cannot, I was duped into buying the Legendary and Mythic as well but I just bought ODST anyway so I have all the maps. You can transfer content licenses to different consoles though.