3v3 is a cooperative game mode where you and two team mates must defeat your three opponents in order to gain a point. The first team to three points, wins. You can make a 3 person team of your friends via a matchmaking group name.
Something else that differs 3v3 from 1v1, is that there are 4 Item Pedestals randomly spread across the map, as well as a Quick Sand. In addition, obstacles like rocks are also common in this game mode, unlike 1v1.
As with 1v1, you get 10 arena points if you win the match, but lose 10 points you lose.
Before each game, 3 players on each side spawn in the top left, and the bottom right.
3v3 arena mode is currently removed, and replaced with 2v2/browars, which is basically the same but with two people against two people.