The description for this is misleading. You need to kill at least one of each class within I believe 4 seconds. If you succeed, the Trophy will pop right then.
I recommend choosing the Control game mode. Your best chance will be on a small map, not those larger maps where people hop in their Sparrows to get around. You may well get this eventually through normal play, however to increase your chances you should camp near a common flag that your team owns, not moving and out of sight as best you can, and wait to see the flag being taken by the other team. Wait another few seconds to let more of their team arrive, then pop out and drop a super ability on them, and clean up with your shotgun and melee as needed.
If all classes were chosen equally by the other team, I calculate (hopefully correctly!) that the odds of getting this from a triple kill will be 22%, and from a quadruple kill 44% (credit to ElectroSpecter). In practice there are currently fewer Titans than other classes, so the odds will be lower. I don't know the exact ratio of classes chosen, so let's say it might be a ballpark estimate of 1 in 7 to get this from a triple kill. Good luck!