Hello,
Sorry for my bad english.
Last night we have launched a competitive game with 2 friends. In the lobby, we were 5vs5, but 2 guys before launching the party quit. The competitive party will launch in 3vs5… Impossible to play chapel in attacker team in 3vs5 so the only solution is to quit the game and have a penalty. 
I think the game will not launch in 3vs5.
Best regards.
The game will not launch if anyone fails to join the game, but if they join and then leaves, the game will continue, after a few minutes the game will notify all the players by chat, you can leave without penalties.
We’ll stay few minutes (5 maybe) but nothing, no messages tell us we can leave without penalties. I check this to the future.
Thx 
Hello! Uri is correct when players disconnect from a matchmaking game a message does appear informing other players they can leave without getting a penalty. 