It's about leveling the playing field, and reducing queue times. Look, I don't mind losing. In fact, I lose all the time. I have never played against a 3 stack. I play solo, or with one friend. The point is that the matchmaking isn't going well for the 3 stack team. They have to wait longer. This is because very few people want to join up with 2 friends, get smoked by pro teams, and have an awful time blaming each other for not doing this or that. The reduction in population of 3 player teams means those who want to will face the same teams over and over, or spend 20 minutes queueing for a match. By forcing 3 strong players to only play as a duo, or all 3 play as solo, you're able to spread out that talent to other teams and give less skilled players a chance to play along side them. Or, if they insist on only playing with their pro friends, you eliminate them from the pool altogether. Which is fine! They make up a small portion of the players anyway, so good riddance!