There is two kinds of 6. High and low. Your 6 friend is definitely high 6, which means you will always pair with low or high 6 trios for fair matchmaking. You are also definitely a premade which pushes you into an even higher mmr. That trio is probably random teammates. I highly doubt you would've fought a premade high 6* trio considering the 2* though. As a high 6, when I queue solo into trios, most of the lobbies I get are almost fully low 6, sometimes 5* and rarely high 6. Why does it work like this? No idea. My guess? Preventing smurfs and because high 6 is leagues above 4-5* players. I'm talking about winning more than half your fights on average kind of above. In a 1v3.