To determine how much each box of strawberries cost, we can start by dividing the total sale price by the number of boxes.
The total price for the 3 boxes is $42. To find the price of each box:
\[ \text{Price per box} = \frac{\text{Total price}}{\text{Number of boxes}} = \frac{42}{3} = 14 \]
Therefore, each box of strawberries cost $14.