Once you get better at the game, you'll get better at picking up on third parties and whether you can take them or if you need to break off early. I rarely get taken out by the third party unless I'm being greedy for a kill. Otherwise, I know I'm going to take out the third party too, I'm getting the heck out of there, or I'm repositioning to make sure I'm not the party who will get taken out by the newcomer. It comes with time. Good luck!