Allright, let me get straight to the point. None of my friends (or at least not many of them) play halo 3 anymore and the ones who do are obsessed with matchmaking and other stuff. I am a huge fan of custom games and it's very hard to find others who are because, well, their all playing custom games. So here's the point of this. This website looks like it's filled with hundreds of serious custom game addicts who play mainly halo 3. so if anyone wants to, send me a friends invite and let me in on your custom games.
Check out the Customs Lobby, or consider joining the Testers Guild - you can grab loads of customs with a little bit of sustained effort in those forums. Good luck, sure you'll be able to find some games.
What games are your friends playing? They're probably playing all the half-arsed games that were brought out before xmas. They'll all come crawling back, don't worry. If you need campaign help, send me an FR. GT: EpicFishFingers (I am British though, so that might affect when I can help you)
I still play a lot and i like customs, but my FL is full. I'm sure with a little bit of effort, you can find a lot of people willing to play customs with you.
Well, I'm off playing my games that I got for Christmas and all, but soon enough, I will start playing Haloo 3 again, especially with the introduction of the new map pack that will come out relatively soon. Nothing can really compete with the playability factor and greatness of Halo 3's custom games.