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This achievement can be earned at any time during your playthrough after Mission 3. Please keep in mind combat-related achievements only work on enemies in a hostile environment; civilians and policemen do not count.

You will need to unlock and fully upgrade the "Nanoblade" and "Focus Enhancement" augmentations.

Head to the underground Russian casino and make some noise so that you're in hostile combat. Now activate "Focus Enhancement" and aim the Nanoblade at any of the mobsters. They should be grouped up at this point, so just shoot your blade at one of them and the explosion should wipe out three enemies at once, thus unlocking the achievement.