Explanation:
Look at each row individually.
The first row starts with 2 blue squares, then increases to 4 blue squares and finally has 6 blue squares. Meaning that each move along the row increases the number of blue squares by 2.

The third row starts with 3 blue squares, then increases to 6 blue squares and finally has 9 blue squares. Meaning that each move along the row increases the number of blue squares by 3.

The second row starts with 1 blue square and then increases to 2 blue squares. So, the last box must contain 3 blue squares, because each move along the row increases the number of blue squares by 1.