If we were writing all $1000$ numbers from $0$ to $999$ as three-digit numbers, then each digit appears the same number of times, that is $300$ times ($100$ times in each position). Now the difference is that we write one more digit $1$ (for $1000$ instead of $0$) and a lot fewer digits $0$ (all the leading digits). Hence the digit $1$ occurs the most, the digit $0$ the least.