This solution relates to actually getting a Trait to level 3 in-game rather than how to unlock Traits and the requirements in the first place, as B3AN's solution does a good job of that.
The clear point to make is that you don't need to get a certain number of kills. The Trait levels are based on points and, obviously, not dying. To get to level 3, I equipped Bacta Bombs to get points and streaks from healing teammates, and Barrage to increased the chances of multi-kills.
As you can see in the video, I got to Level 2 and then managed a Barrage multi-kill to get to Level 3, however I never got the 6 kills mentioned elsewhere, it's just total points. This means you can totally get to Level 3 by just healing teammates with Bacta and picking up the odd kill.
Extraction is a great game mode as if you are defending you can camp out at a well-protected spot and just pick up a steady stream of points from kills and healing with less risk to yourself. Again, the video shows that.
I hope this helps and good luck.