Sniper triple kills, most effective way?

I only have 3 guns to get to dark spine, those are the 3 snipers. I was thinking of doing the mind numbing throw smoke in stakeout and hold hallway, however I wonder if there is an easier way to do it?

Is Hard-core better or worst for sniper triple kills? Should I camp or just say fuck it and quick scoop until I get to a low SBMM in Hard-core?

Any help would be appreciated :)