I am the third of four children.

I am the youngest of three children. Hope this helps :-) Now that I have noticed the "additional details" the answer should be - I am the third child in a family of four. I guess you could translate your Chinese sentence as "I am the third child in my family" but my Chinese language skills aren't great :-) One more thing, it should be "an elder" not "a elder".

I am the second youngest of four. I have an older brother and older sister. (you could stop there, but for clarity you may add) In total, there are three boys and her.