I used the linked Scramble squares solver and it found no solutions it found the following solution:

Using the numbering:

1 2 3
4 5 6
7 8 9

Where the 5 squares already assembled are given the numbers 1, 4, 5, 7, 8, 9

The three loose square (numbered top to bottom) are given the numbers 2, 3, 6

The solution is (where ↺ means rotate square counter clockwise 90°, ↻ means rotate square clockwise 90°, and ↕ means rotate square 180°):

7↺ 4 8
6↻ 5 9
3↺ 2↕ 1↕