I assume you mean measure the force.
First five drops would be to drop 1-25 in batches of 5. You get the 5 heaviest stones in their respective pools.
6th drop would be a drop among the five stones winning their respective drops. The heaviest stones is placed aside (on the podium as No 1)
7th drop would be finishers No 2 and 3 of drop 6, No 2 and 3 of the initial five drops in which the heaviest stone won and No 2 of drop of the initial give drop in which the runner up of the sixth drop finished first. The top two of drop 7 are the second and third heaviest stones respectively.
Basically the 5 fastest horses in 25 horses problem.
Explanation for drop 7
Since the heaviest stone competed in its initial drop as well as the 6th drop, the 2/3 finishers in both drops are candidates for second and third heaviest. And since the runners up of drop 6 finished first in its initial drop, we need to include the runners up from its initial drop as that could be the third heaviest stone. (Read the horses version online for a clearer explanation)