[doorgunner69]

What I got.

Good old fashioned stuff, still works
dog 17
Cat 7
Rabbit 3

[Alberta's Child]

Lots of smart folks here, but most of you made it too complicated. You didn’t need to solve for the weight of each animal if you only needed to know the combined weight of all three.

The way the problem is written, each animal was weight twice — and only twice. If the total weight of the three weighings in the three combinations of animals was 54 kg and it doesn’t matter how much each one weighs, just divide 54 by 2 and you get 27 kg.