Give a triangle $ABC$ with heights $h_a = 3$ cm, $h_b = 7$ cm and $h_c = d$ cm, where $d$ is an integer. Determine $d$.
1. **Calculate the area of the triangle using the given heights:**
   The area \( S \) of the triangle can be expressed in terms of the base and corresponding height:
   \[
   S = \frac{1}{2} a h_a = \frac{1}{2} b h_b = \frac{1}{2} c h_c
   \]
   Given \( h_a = 3 \) cm and \( h_b = 7 \) cm, we can write:
   \[
   S = \frac{1}{2} a \cdot 3 = \frac{1}{2} b \cdot 7
   \]
   Simplifying these equations, we get:
   \[
   S = \frac{3a}{2} = \frac{7b}{2}
   \]
   Equating the two expressions for \( S \):
   \[
   3a = 7b \implies a = \frac{7b}{3}
   \]

2. **Use the triangle inequality to find the range of \( d \):**
   The triangle inequality states that for any triangle with sides \( a \), \( b \), and \( c \):
   \[
   a + b > c, \quad a + c > b, \quad b + c > a
   \]
   Using the heights, we can derive the inequality for \( \frac{1}{h_a} + \frac{1}{h_b} > \frac{1}{h_c} > \left| \frac{1}{h_a} - \frac{1}{h_b} \right| \):
   \[
   \frac{1}{3} + \frac{1}{7} > \frac{1}{d} > \left| \frac{1}{3} - \frac{1}{7} \right|
   \]

3. **Calculate the bounds for \( \frac{1}{d} \):**
   \[
   \frac{1}{3} + \frac{1}{7} = \frac{7 + 3}{21} = \frac{10}{21}
   \]
   \[
   \left| \frac{1}{3} - \frac{1}{7} \right| = \left| \frac{7 - 3}{21} \right| = \frac{4}{21}
   \]
   Therefore, the inequality becomes:
   \[
   \frac{10}{21} > \frac{1}{d} > \frac{4}{21}
   \]

4. **Solve for \( d \):**
   Taking the reciprocals of the inequalities:
   \[
   \frac{21}{10} < d < \frac{21}{4}
   \]
   Simplifying the bounds:
   \[
   2.1 < d < 5.25
   \]
   Since \( d \) is an integer, the possible values for \( d \) are 3, 4, and 5.

5. **Determine the correct value of \( d \):**
   To ensure the correct value, we need to check if these values satisfy the original area equations. However, since the problem only asks for \( d \) as an integer within the range, we conclude that \( d \) can be any of these values.

The final answer is \( \boxed{ d = 3, 4, \text{ or } 5 } \).