From the message, it's clear that your friend is talking about a cat that has given birth. The cat (referred to as 'Tatze') has had 3 kittens ('3 Junge'). So you can congratulate her on the arrival of three new kittens! Here's a possible message: "Herzlichen Glückwunsch zu den 3 neuen Familienmitgliedern! Die Kleinen werden sicher so süß sein. Ich wünsche euch viel Spaß mit den Küken. Möchte bald mehr sehen!" Translation: "Congratulations on the 3 new family members! The little ones will surely be so cute. I wish you a lot of fun with the kittens. Can't wait to see more!"