All 3 locations:
1. After using the Embalming Fluid, you fall down into a room with water, that's where the first UPV is.
2. You have to get spotted by the guards before entering the large underwater sewers. Your second UPV is next to the opening with the air pocket for the main underwater sewers area, next to a dead diver.
3. You have to sneak around the guards before the large underwater sewers area without getting spotted. Your third UPV is inside the underwater Secret that you unlock because of it.