Surely the question is of two men on a see saw at a time as is conventional rather than a makeshift pivot scale making the question much harder as it is 12 men 11 of which are equal weight and one is lighter but to find out you can only use the seesaw three times
1. Divide into two groups of 6. The heavier group (indicated by their seat touching the ground/lower than other seat) leaves.
2. The remaining 6 are divided into two groups of 3. Once again, the heavier group leaves.
3. Two of the remaining 3 are used for the final test. If they are of equal weight, the one not used is the lighter. Otherwise, the lighter one is the one who is higher in the air.