It's a present participle verb (if used as an adjective or a gerund if used as a noun. ) we use them in English but they are more common in Russian, i think.
"A cat weighing. ..", although uncommon, is proper. You just won't hear it in colloquial conversation too much, but maybe more in Downton Abbey, or Monty Python.
Hmmm. I think "a 3kg weighing cat" has a different meaning. I have an image of a cat who constantly weighs objects that are 3kg as if it was his job. It doesn't sound like the CAT weighs 3kg. The cat becomes the actor of "weighing" instead of it being a passive quality. Hope this answer is helpful for the 3 people who had this question...
I put килограмм instead of килограмма. I was not warned of a typo..
Sometimes I'm totally wrong for just one character, or excentric but not wrong word order.. However other times I'm congratulated with a "perfect" without a warning of my typo or without showing me the really accurate right answer.
Скажите, пожалуйста, is there a rule for some typos or others? I understand that if just one wrong letter implies a wrong case, or the confusion of plural and singular, it verb tense, then that typo is enough for everything to be OK, and if the typo is a spelling or typing mistake that doesn't change tense, aspect, gender, number, etc it is acceptable. But it would still be good to receive a warning, as it sometimes happens and sometimes doesn't