The Faulty Combination Lock

A combination lock with three dials, each numbered 1 through 8, is defective in that you only need to get two of the numbers right to open the lock. (For example, suppose the true combination is 4-2-7. Then 4-2-7 would open th lock but so would 4-2-5, 4-2-2 , 8-2-7 or 4-6-7. But not 2-4-7)

What is the minimum number of (Three-number) combinations you need to try in order to be sure of opening the lock?