Our eldest is a girl, the middle one is a boy, and the youngest one is a girl too.
Our first and last are girls and we have a boy in the middle.
この場合、次のような言い方ができますよ。
ーOur eldest is a girl, the middle one is a boy, and the youngest one is a girl too.
「うちの一番上の子は女の子で、真ん中が男の子、一番下がまた女の子です」
ーOur first and last are girls and we have a boy in the middle.
「1番目と三番目が女の子で、真ん中に男の子がいます」
例：
I have three children. The eldest is a girl, the middle one is a boy, and the youngest one is a girl too.
「私は3人子供がいて、一番上が女の子で、真ん中が男の子、一番下も女の子です」
ご参考まで！
カナダ 
