Hi, Joel, you did a good job!! Just please remember we use measuring word to modify nouns quite often which is pretty like the articles, THE, A, AN, in English:) So in that case you put there, it would be better if you say: 我是妈妈的第三个孩子。 Furthermore, 三 is a cardinal number, 第三is an ordinal number, while 第三个 is what you can put before a noun as an adjective. Hope it helps! Have a nice day! Candy

你是老三。

我是妈妈生的第三个孩子；或者:家里的孩子中，我排第三；或者:我是家中的老仨

少了量词“个”，英语中没有量词，中文中需要注意

Grammatically correct and translation precise but it is not the natural way to say. 我是第三个孩子, 我是排第三 or even more colloquially, 我是老三.