You should be able to find the inlet without taking them apart.
1. Turn off all water usage in the house.
2. Heat the 3 possible inlet lines for about 4 inches next to the concrete with a torch or heat gun, but keep below the boiling point.
3. While checking the three possible inlet lines with your hand, have someone run some HOT water.
4. The one that gets cold first where it comes out of the concrete is the supply line.