What is the differecne between, lets say those sentences?
もの is used for tangible objects. 面白いもの could refer to some interesting object.
こと is used for intangible objects such as concepts. 面白いこと could refer to some interesting concept.
の is used to say, for your example "interesting one." Sometimes, こと and の and transposable.
Of course, there is more nuance to all three, but what I said should explain your sentence:
”Please tell me the interesting one/‘part.’”