Puzzle n°23 (***) - In 3 weighings only

I had been given 12 samples of cat kibble,
but the sealed bags were not printed with anything to identify the type of food inside.
And they all looked exactly the same !

However, I had been told that among the 12 samples,
there was a sample of kitten kibble, all the others containing one single sort of adult kibble in equal quantities.
Today, I needed to open the kitten bag,
but how could I identify it without opening all the bags first ?

Hopefully, I knew that this kitten sample had a weight a bit different from the adult ones,
and that the 11 adult samples were all exactly of the same weight.
However, I didn't remember whether the kitten bag was heavier or lighter than the adult ones.

All I had was a Roberval-type scale with two sides, as the one pictured here.
But I didn't have any measuring weights.
How could I identify the kitten sample for sure in only 3 weighings,
and know whether it was heavier or lighter than the others ?

... I'm giving up ...