push them together put the book on top of them all at once and turn them on either at the same time or one at a time the readings will be different telling you which is which
No it's more simple.
You test only two and record the result,
A & B = 1 kg difference B is more than A.
B & C = 2 kg difference you now know A.
A & C = 1 kg difference C is less than A.
You can determine which scale is A simply by selecting two scales at random and seeing what their results are then compare.
Scale A will always be less or more than another scale.
If you happen to select B and C itll have a difference of 2kg so you immediately know which is scale A. You don't even need to test A, you already know which is A.
Testing B & C is 100% success.
Testing A & B or A & C have 33% chance failure.