You should buy a map in Deling City. That makes makes the whole thing easier...

Just enter the tomb and walk up one screen to find the sword with the number on it. Now you can leave and go back to Deling City and talk to the guy in front of the mansion and tell him the code (the ones digit first, then the tens digit and finally the hundreds digit).

But you have already begun the Brothers' side quest so you may want to finish it. After releasing the chain and letting the water in you have to go to the western chamber of the tomb. There you have to change the direction of the waterwheel. Now (go out and save before you) go to the center of the tomb (straight north from the entrance) and fight the Brothers to get them as a GF.