Is 下个星期三 (xiàge xīngqīsān) = "next Wednesday" ambiguous like in English?
下个星期三 would only refer to Wednesday, next week. There is no way you would use 下个星期三 to refer to Wednesday this week, like the one that is coming two days from now, that would just be 星期三.

There's a question on Baidu Zhidao that, kind of(?), asks this.


  下个星期三要多少天?


with answers like:


  8天


and


  七天 


Perhaps Larry David would be disappointed though.