The combination lock has three wheels, each numbered from 0 to 9. what is the probability that someone randomly guesses the correct combination in one attempt?

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.