「3人目が生まれたの?」は
You had your third child?
You had a third kid?
のように言えます。
例:
You had a third kid already? It seemed like you just had your second not long ago.
「もう3人目が生まれたの?この間、2人目が生まれたところだと思ったけど」
ご参考まで!
**Did you have your third baby?**
**So you had your third?**
**Is this your third baby?**
「3人目が生まれたの?」「3人目できたの?」という驚きや確認のニュアンスは、英語では **Did you have your third (baby)?** や **Is this your third?** などの自然な口語表現で伝えられます。