I like playing 3v3 arena and I hope everybody can compete in the spirit of fair play. Several MMR exploits have existed for a long time. However, Blizzard didn't fix them. This is one of them that I know. Anyone who can report this to Blizzard, please do it!
This works on 2v2, 3v3 and 5v5.
For example, if we use the MMR exploit in 3v3, we need 7 characters. Then we create 2 arena teams. Let's say they are team A, team B and character C.
We have A1, A2, A3 in team A and B1, B2, B3 in team B. Before we get started, we need to know which character has the highest individual MMR in each team. Let's say A1 and B1. Here, individual MMR means the MMR of that character. We can see it at the end of a 1 vs 3 match if we only let that character enter arena.
Then let me show you how it works:
Redo step 4, 5 and 6. (You should always let the party leader be the member with the highest individual MMR in that team. Here we have A1 and B1 at the beginning. Also, if you met another random team, only A1 and B1 enter the arena, and they must enter. Otherwise, MMR would be screwed up)
At the end, MMR will be very high for both team A and team B. Then we can remake team A and team B, and we can easily get two high-rated teams with 50% win-loss ratio.
Here are three ways to fix it:
1. No free rating. For example, if only two opponents entered a 3v3 arena, your team will not be given any free rating or free MMR increase. (But the rating of your opponent's team will decrease as punishment of incomplete team members)
2. Tie MMR at team level. For example, if we have team A with 1800 MMR at the beginning of the season. Then A1 left team A and get rank 1 in another team. At the end of the season, A1 goes back to team A. In this case, MMR of team A will still be exact 1800, whatever A1, A2 and A3 did during this season.
3. If you win, only you win, if you lose, everybody loses. For example, if I enter a 3v3 arena, but my teammates are all DCed. And I kill all the opponents and I win. In this case, only my MMR increases, nothing happens to my teammates. On the other hand, If I enter a 3v3 arena and my teammates are still DCed. Then I lost. In this case, everybody in my team gets MMR decrease and everythig decreases. (team rating, personal rating). This rule applys when someone in your team doesn't enter an arena for whatever reason!