look on the inside of the case at the back of the combination mech. should be a leaver or switch that you move/press and change the code. Then put back to original position.
Your lock has three numbers to use. Am I right? In that case try triple zero, triple one, triple two etc. up to triple nine, you might find one of them works. I forgot the number of my Samsonite brief case but triple two did it! Good luck ; i hope you'll let us know how it works out.