According to images shown from multiplayer matches in Black Ops 3, if you join a game in progress and the team you’re placed on loses that match, the loss stat won’t count against you.
The message in game says “Loss Stat Preveneted. Joined game in progress.”
Vahn also confirmed this feature.
It's important to fill games. Just a small way to make it less painful. Inspired by the Wins Before KD crew. + https://t.co/enenUdlo7o
— David Vonderhaar (@DavidVonderhaar) November 5, 2015
SOURCE: Reddit user S2_Tact via @ImDehV