SOLVE this riddle

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.

but that would take longer

You are only testing two how would testing two take longer?

You realize placing the book evenly over all three would throw off the weight since the other two scales would take some if it's weight.

placing the book evenly wouldnt throw off the weight as you clearly stated all 3 scales are identical so putting them together would just create a larger scale so the results would be the same as weighing them individually

I am 100% positive it would throw off the weight putting them together.