When the movie is half over, popcorn is sold at half price.
The full price of popcorn is $3.50, so the half price is: \[ \text{Half price popcorn} = \frac{3.50}{2} = 1.75 \]
The price of one drink is $1.35.
Now, we can calculate the total cost for purchasing 3 orders of popcorn and 3 drinks.
Total cost for popcorn: \[ \text{Cost of 3 popcorns} = 3 \times 1.75 = 5.25 \]
Total cost for drinks: \[ \text{Cost of 3 drinks} = 3 \times 1.35 = 4.05 \]
Total cost: \[ \text{Total cost} = \text{Cost of popcorn} + \text{Cost of drinks} = 5.25 + 4.05 = 9.30 \]
Thus, the total cost for purchasing 3 orders of popcorn and 3 drinks when the movie is half over is: \[ \boxed{9.30} \]