Step 1: Understanding the harmonic vibration.
In the 3rd harmonic, the number of antinodes is 3. The positions of the antinodes for the 3rd harmonic are given by the formula:
\[
x = \frac{L}{4}, \frac{3L}{4}, \frac{5L}{4}
\]
where \( L \) is the length of the sonometer wire. These are the positions where the maximum displacement occurs.
Step 2: Conclusion.
Thus, the antinodes are formed at \( \frac{L}{4}, \frac{3L}{4}, \frac{5L}{4} \), corresponding to option (D).