Based on the message, your friend is talking about a cat (Tatze/Katze) that had 3 kittens (3 Junge). It appears that someone named Lisa shared on Instagram that her cat had 3 babies, and your friend thinks they look very cute and wants a cat too. So if you want to congratulate your friend, you would be congratulating her on 3 new kittens. However, note that the message suggests these might be Lisa's kittens, not directly your friend's, as your friend seems to be telling you about Lisa's post.