I have never been disconnected from a game before, so cant really comment on that.
This is completely and 100% not true. Team comp does not mean an auto win/lose. I have had games where either my team or the other team had no tanks, multiple tanks, no support, or multiple support, and won/lose those. Its all about player skill. If its not about player skill you might as well quit within the first 15 seconds based on your team comp, because the outcome is decided right?
Also, the only time i have had 3 people disconnect/quit is when that team was getting stomped and they didnt want to stick around any longer. Sure your going to have games here and there that are 4v5, but that accounts for only a small portion of games