The smallest three positive proper divisors of an integer n are $d_1 < d_2 < d_3$ and they satisfy $d_1 + d_2 + d_3 = 57$. Find the sum of the possible values of $d_2$.
1. Given that the smallest three positive proper divisors of an integer \( n \) are \( d_1 < d_2 < d_3 \) and they satisfy \( d_1 + d_2 + d_3 = 57 \).
2. Since \( d_1 \) is the smallest positive proper divisor, it must be \( d_1 = 1 \).
3. Therefore, we have \( 1 + d_2 + d_3 = 57 \), which simplifies to \( d_2 + d_3 = 56 \).
4. \( d_2 \) must be a prime number. If \( d_2 \) were not a prime number, there would be another divisor between \( d_1 \) and \( d_2 \), contradicting the fact that \( d_2 \) is the second smallest divisor.
5. Since \( d_2 + d_3 = 56 \) and \( d_2 < d_3 \), we have \( 2d_2 < 56 \), which implies \( d_2 < 28 \).
6. The possible prime values for \( d_2 \) less than 28 are \( \{2, 3, 5, 7, 11, 13, 17, 19, 23\} \).

Now, we check each possible value of \( d_2 \):

- If \( d_2 = 2 \), then \( d_3 = 56 - 2 = 54 \). Since \( 54 \) is not a prime number and has divisors other than 1 and itself, it cannot be the third smallest positive divisor.
- If \( d_2 = 3 \), then \( d_3 = 56 - 3 = 53 \). Since \( 53 \) is a prime number, it is possible.
- If \( d_2 = 5 \), then \( d_3 = 56 - 5 = 51 \). Since \( 51 \) is not a prime number and has divisors other than 1 and itself, it cannot be the third smallest positive divisor.
- If \( d_2 = 7 \), then \( d_3 = 56 - 7 = 49 \). Since \( 49 = 7^2 \), it is possible.
- If \( d_2 = 11 \), then \( d_3 = 56 - 11 = 45 \). Since \( 45 \) is not a prime number and has divisors other than 1 and itself, it cannot be the third smallest positive divisor.
- If \( d_2 = 13 \), then \( d_3 = 56 - 13 = 43 \). Since \( 43 \) is a prime number, it is possible.
- If \( d_2 = 17 \), then \( d_3 = 56 - 17 = 39 \). Since \( 39 \) is not a prime number and has divisors other than 1 and itself, it cannot be the third smallest positive divisor.
- If \( d_2 = 19 \), then \( d_3 = 56 - 19 = 37 \). Since \( 37 \) is a prime number, it is possible.
- If \( d_2 = 23 \), then \( d_3 = 56 - 23 = 33 \). Since \( 33 \) is not a prime number and has divisors other than 1 and itself, it cannot be the third smallest positive divisor.

From the analysis above, the possible values \( d_2 \) can take are \( 3, 7, 13, \) and \( 19 \).

Therefore, the sum of the possible values of \( d_2 \) is:
\[ 3 + 7 + 13 + 19 = 42 \]

The final answer is \( \boxed{42} \).