For a string vibrating in the third harmonic (n = 3), the number of nodes and antinodes can be determined using the following relationships:
- The number of nodes in the third harmonic is \( n + 1 \), where \( n \) is the harmonic number.
- The number of antinodes is \( n \).
Thus, for the third harmonic:
- Number of nodes = \( 3 + 1 = 4 \),
- Number of antinodes = \( 3 \).
Therefore, the correct answer is:
\[
\boxed{(B) \, 4 \, \text{nodes,} \, 3 \, \text{antinodes}}
\]