You can use the reload checkpoint method to get this once you find the first guy by the water during the second part of the Geothermal Valley.
Or the first place you can get three without the reloading is in the Flooded Archives, near the Greek Fire Depot campsite. While trying to blow up the statue in the center you can come across a group of Trinity soldiers to grab, South of the campsite though a flooded corridor. Just grab three while the others are turned and you will drown them and unlock the achievement. Shouldn't have any troubles with this here.