Based on the clues:
- Keshav is second tallest but younger than Rekha.
- Parul is taller but younger than Megha.
- Rekha and Megha are the same age, with Rekha being the tallest.
- Nikku is taller than Parul and older than Rekha.
Age ranking (descending):
- Rekha (tallest, therefore oldest).
- Keshav (second tallest, younger than Rekha).
- Nikku (older than Rekha, taller than Parul).
- Parul (younger than Megha, Keshav).
- Megha (youngest).
Therefore, \boxed{\text{None of these}} is third in age.