|Sorry, you failed!
As the adage goes, if at first you don't succeed, try, try again!
Carefully check your efforts for finding the third number, to see if you have made a mistake. On many locks, there will be two numbers that appear to be the correct third number. If so, try again with the other third number. (In most cases, there were two very close possibilities for the last number, and you have had the misfortune of selecting the wrong one.)
If the third number appears to be correct, try the 64 combinations again. You may have missed a number along the way, or turned past it. Even if you dial the correct combination and are a little sloppy when you dial, the lock may not open. Also, some locks are very "picky" and don't always open, even if you dial carefully. A second pass may work.
Try practicing on a lock for which you have the combination. It appears that locks in larger stores that turn over their inventories rapidly, such as Home Depot or Target, will open using these methods. Check the serial numbers. Have a friend remove the combination sticker, open the lock using the correct combination (to make sure it works), close the lock and spin the dial a few times. Find the last number yourself and let your friend verify it. If wrong, try to figure out why you picked the wrong last number. Then, try these methods using the correct last number. Don't frustrate yourself by dialing combinations that have a wrong last number!