Namita is taller than Priya but not as tall as Madhuri. Reena is not taller than Namita but is not the shortest. Who among them is the third tallest?

Let's represent the heights in decreasing order based on the statements:
1. "Namita is taller than Priya but not as tall as Madhuri" $\to \text{Madhuri}>\text{Namita}>\text{Priya}$.
2. "Reena is not taller than Namita but is not the shortest" $\to$ Reena must be shorter than Namita ($\text{Namita}>\text{Reena}$) but taller than the shortest person, who is Priya ($\text{Reena}>\text{Priya}$).

Combining both conditions, the final order from tallest to shortest is:

$$\text{Madhuri}>\text{Namita}>\text{Reena}>\text{Priya}$$

Counting from the tallest:
1st tallest: Madhuri
2nd tallest: Namita
3rd tallest: Reena
4th tallest: Priya

Thus, Reena is the third tallest.