A nine-digit number has the form $\overline{6ABCDEFG3}$, where every three consecutive digits sum to $13$. Find $D$.

[i]Proposed by Levi Iszler[/i]
Given a nine-digit number of the form $\overline{6ABCDEFG3}$, where every three consecutive digits sum to $13$, we need to find the value of $D$.

1. **Set up the equations based on the given condition:**
   - The first three digits sum to $13$: $6 + A + B = 13$
   - The next three digits sum to $13$: $A + B + C = 13$
   - The next three digits sum to $13$: $B + C + D = 13$
   - The next three digits sum to $13$: $C + D + E = 13$
   - The next three digits sum to $13$: $D + E + F = 13$
   - The next three digits sum to $13$: $E + F + G = 13$
   - The last three digits sum to $13$: $F + G + 3 = 13$

2. **Solve the first equation:**
   \[
   6 + A + B = 13 \implies A + B = 7
   \]

3. **Solve the second equation:**
   \[
   A + B + C = 13 \implies 7 + C = 13 \implies C = 6
   \]

4. **Substitute $C = 6$ into the third equation:**
   \[
   B + 6 + D = 13 \implies B + D = 7
   \]

5. **Substitute $C = 6$ into the fourth equation:**
   \[
   6 + D + E = 13 \implies D + E = 7
   \]

6. **Substitute $D + E = 7$ into the fifth equation:**
   \[
   D + E + F = 13 \implies 7 + F = 13 \implies F = 6
   \]

7. **Substitute $F = 6$ into the sixth equation:**
   \[
   E + 6 + G = 13 \implies E + G = 7
   \]

8. **Substitute $F = 6$ into the seventh equation:**
   \[
   6 + G + 3 = 13 \implies G + 9 = 13 \implies G = 4
   \]

9. **Substitute $G = 4$ into $E + G = 7$:**
   \[
   E + 4 = 7 \implies E = 3
   \]

10. **Substitute $E = 3$ into $D + E = 7$:**
    \[
    D + 3 = 7 \implies D = 4
    \]

Thus, the value of $D$ is $\boxed{4}$.