Hi jassygram,
It might be random, I'm not sure. Did you take a peek at MaG's walkthrough? Here's what it says:
You are given 3 chances to use the scale to find out.
Place 3 mice on each pan. Press the scale button on top to weigh. The bottom button is to reset the puzzle.
A. If they weigh the same, use the remaining 3: one on each pan and one in reserve. Check which is lighter.
If the scales are even, the one in reserve is the lightest.
B. If one is lighter, take off the heavier one and check the 3 mice that are lighter.
Use the remaining 3: one on each pan and one in reserve. Check which is lighter.
If the scales are even, the one in reserve is the lightest.