 Kevin writes the multiples of three from $1$ to $100$ on the whiteboard. How many digits does he write?
1. First, we need to determine how many multiples of 3 are there between 1 and 100. The smallest multiple of 3 in this range is 3, and the largest is 99. We can find the number of multiples of 3 by solving the equation \(3n \leq 100\), where \(n\) is the number of multiples.
   \[
   3n \leq 99 \implies n \leq \frac{99}{3} \implies n \leq 33
   \]
   Therefore, there are 33 multiples of 3 between 1 and 100.

2. Next, we categorize these multiples based on the number of digits they have. The multiples of 3 from 1 to 9 are single-digit numbers, and the multiples of 3 from 12 to 99 are two-digit numbers.

3. The single-digit multiples of 3 are: 3, 6, and 9. There are 3 such numbers.

4. The two-digit multiples of 3 start from 12 and go up to 99. To find the number of two-digit multiples, we subtract the number of single-digit multiples from the total number of multiples:
   \[
   33 - 3 = 30
   \]
   So, there are 30 two-digit multiples of 3.

5. Now, we calculate the total number of digits written by Kevin. Each single-digit number contributes 1 digit, and each two-digit number contributes 2 digits. Therefore, the total number of digits is:
   \[
   3 \cdot 1 + 30 \cdot 2 = 3 + 60 = 63
   \]

The final answer is \(\boxed{63}\).