We've got Forza horizon 3 and gold live but how do we have multiple players?

They would have to be playing over Live with 2 different consoles. There is no local multiplayer for that game.

Eligible players will receive codes to download the Warthog via an Xbox Live Message, and codes will begin to be sent out during the week of Sept. 5. In addition, players who have not played an Xbox One Halo game will still have the chance to earn the Warthog
via a special #Forzathon event coming to Forza Horizon 3 in October.

For more info:
news.xbox.com/.../forza-horizon-3-gone-gold-halo-warthog-announcement

I played forza horizon then forza 3 beat horizon then got 3 played 3 until I got to year 3 in season racing went back to horizon and got the achievement and cars tried it on a different profile and it wont give me the achievement so ignore all this garbage
about swapping save devices and what not there is a bug or 2 in horizon and that is one of them so it may or may not give you it there isn't any thing you can do about I have tried all theories on how to get it and non of them work

Hi Francesco,

Yes, Forza Horizon 3 offers seamless Cross-Play support powered by Xbox Live, linking players on Xbox One and Windows 10 PC together in the same game session. This is all confirmed and explained

here.

You would still need Xbox Live Gold to play online on Xbox though. However, you don’t need Xbox Live Gold to play online on PC.

Hope this helps 😊

"The M12S Warthog CST will be available at launch for Forza Horizon 3, and will be sent as a free download to everyone who has played either Halo 5: Guardians or Halo: The Master Chief Collection on Xbox One. Eligible players will receive codes to download
the Warthog via an Xbox Live Message, and codes will begin to be sent out during the week of Sept. 5. In addition, players who have not played an Xbox One Halo game will still have the chance to earn the Warthog via a special #Forzathon event coming to Forza
Horizon 3 in October."

Source: news.xbox.com/.../forza-horizon-3-gone-gold-halo-warthog-announcement

Codes are usually sent out in batches and given that you have played one of the two games (like you mentioned) you should receive or have received a message from Xbox Live.

If you have ever received a message from Xbox Live in the past, it may be worth going through your message list to see if there's an unread message from Xbox Live due to the recent issues surrounding the Xbox Messages on the Xbox One (new messages not appearing
at the top.)

To answer your question properly, there is no specific timeframe for when you will receive a code. Hopefully you'll grab it soon ready for the 27th!