When the movie is half over, popcorn sells for half price.
The full price for popcorn is $3.50. Therefore, the half price is: \[ \text{Half price of popcorn} = \frac{3.50}{2} = 1.75 \]
Next, we calculate the cost for 3 orders of popcorn at the half price: \[ \text{Cost for 3 orders of popcorn} = 3 \times 1.75 = 5.25 \]
The price for drinks remains the same at $1.35 each. So, the cost for 3 drinks is: \[ \text{Cost for 3 drinks} = 3 \times 1.35 = 4.05 \]
Now, we add the total cost for the popcorn and the drinks: \[ \text{Total cost} = 5.25 + 4.05 = 9.30 \]
Therefore, the total cost for purchasing 3 orders of popcorn and 3 drinks when the movie is half over is: \[ \boxed{9.30} \]