What and which difference

It is difficult to know what idioms are in fashion and what are not
It is difficult to know which idioms are in fashion and which are not.
It is difficult to know what idioms are in fashion and which are not.
Are all the three sentences correct?

I got this on this site.(http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv175.shtml) Please correct if they are wrong.
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