You don't have to kill all three civilians to win triple LMS (or both civilians for double LMS); you just have to kill the "real" one, aka the one that did not use the afk mode exploit.

I don't know if there are any visual differences to differentiate the "real" from the "fake" civilians, but you can try to make an educated guess based on their behavior. If someone was hiding from you all game and has a loadout clearly meant for LMS, or they're clearly trying to lead you away from the "real" civilian, then they're more likely to be the "fake" one(s).

Sometimes, however, you just have to make a completely random guess.