Do people still play the Multiplayer on Halo 3 actively?
Or am I gonna only be able to get the Campaign Achievements?
It is still active. Searching times are not that bad either unless you search in a mixed skill party, then you can expect to wait awhile for a game. I played for a few days about a week ago.
-Play action sack for your first game, you’ll get an xp point in the list, win or lose but only if its your first game.
I was playing it about 10 mins ago.
Right now, it’s plenty active. Tons of people on. 
Just played, there’s a decent amount of people on, you might want to start playing before the community decreases.
It was pretty weak for quite a while, but for the past few weeks, there’s tons of people on.