So it seems you have to be the last fighter and do it yourself. My team mate just got all 3 kills after we were both dead and i didn't get credit.
Doing this 300 times by yourself seems a little odd. -Might count if you're in an actual party of 3 though, i was just matchmaking and purposely picking second spot to make this happen so that if the first guy dies to the first enemy, then i can pretty much fail before killing their first guy too and hope the 3rd team mate can do the carry, finish the first opponent and solo 2 of them. Which he did.
Anyways. This has to be tested in a real party. Otherwise this title is one of the most insane one. (You'd have to spend your life always selecting 3rd spot and hoping your team are too bad to even kill the first opponent).
Least now i can stop selecting the middle position as i know it's not going to do anything in solo matchmaking.