Both the Nanoblade and Focus Enhancement experimental augmentations need to be unlocked for this achievement. The Nanoblade also needs to be upgraded to have the "Explosive Heat Blade" feature.
A good location to get the achievement is Otar Botkoveli's underground casino in Prague. Once the Dvali henchmen become hostile, there will more than likely be three close together that you can use the explosive blade on. First, activate the Focus Enhancement, then shoot the blade into the center of the enemies. As long as they are close enough together and at least three get hit by the blast, you will get the achievement.
The action of killing three hostiles with one Nanoblade explosion itself also grants a bonus named Master Blaster (5XP), in addition to the usual Close Shave (10XP) for every enemy killed in such explosions.