It's pretty much being at the right place at the right time. If you're host, then the chances of hitting one are EXTREMELY low, so bear that in mind. Try playing with some good players - this is a huge help, as keeping the opposition spawn trapped will open many more oppurtunities where the enemies may be lined up.
NEVER stay back. Try and rush wherever possible. Use air support, frag grenades, whatever you need, to keep them stuck in their spawn. If you're playing core gametypes then, obviously, use Stopping Power.
I would recommend Ground War, Domination, and Hardcore Headquarters:
Ground War = More players, so more chances to hit triples, but you may have a harder time getting them spawn trapped, so you need a very good team.
Domination = The spawns are controlled by the flags. Capture B flag and one other, never capture all three. These matches last for quite a while, which means you get more time to spawn trap them.
HCHQ = Less health, so almost no hitmarkers. Pretty random spawning on some maps though, but there are also some good spawn traps, e.g. Bog, District, Pipeline.
If you want any more help, message me, and I'll give you any info you need about sniping or spawn trapping. I'm an expert at this shit.