Was it King Shlomo or Chizkiyahu and his colleagues? And when did Chizkiahu live?
The boring answer is that King Solomon wrote these three books. Then some people will argue the academic view would suggest otherwise.
A wise man will understand that selfhood is not a matter of flesh but a matter of spirit.
The spirit of the Wise King of Israel is the abstract persona which hovers above these texts.
A more advanced answer is that you will have to read them and internalize them and discover it is nothing but your own soul which wrote these books.
I can also suggest that these holy books were spontaneously teleported into your mind from a primordial place.
At the end of the day the universe has one single author and that is what these books will teach you. It is He who ultimately wrote these books into His narrative.