When you first enter the sewers, equip your smoke grenade and walk down the path that you are supposed to take. When the three enemies appear, position yourself close by and shoot the smoke grenade in the middle of all of them. Wait until the smoke clears and continue on. This is easier than trailing them one by one.

Make sure that the three soldiers are walking on water, then shoot a Sticky Shocker on the water. The soldiers will get electrocuted. Make sure Sam is on a platform before doing this.