Don't think anyone knows for sure, but I personally believe what you said is true. If the person leaves, you're kills on them don't count. Because I've had an achievement pop when in the end of the game it was 1 on 1, me vs someone, I curb stomped them whenever I downed them (it was 1v1 for like practically the whole game) and it popped. So the 3 on 3 or more doesn't matter.
If you regularly play diablo on xbox one add me
GT: Leone Phelps