Since it is not mentioned in the problem, we will assume that repeating of number is allowed. For the first wheel, you have 10 possible digits. For the second wheel, you have 10 possible digits. For the third wheel, you have 10 possible digits. 10 x 10 x 10 = 1000 The probability that someone randomly guesses the correct combination in his first attempt is 1/1000.
If this was without repeat, then 10*9*8 = 720 The probability that someone randomly guesses the correct combination in his first attempt is 1/720.
