All 3 compounds are basic structures in Organic Chemistry, more precise: Aromatic Chemistry, as all of them have a benzene ring (having a benzene in compound is definition of Aromatic compound). That gives us first difference between them, benzene is the simplest compound of all of them, cyclic hydrocarbon in shape of hexagon, with 3 double bonds (C6H6). Next compound, Phenol gives itself away by its name: Phen-ol, where -ol ending tells us that it is an alcohol. Thus, it is the same benzene ring, but now with an alcohol group added to it (also called Hydroxy group), sometimes abbreviated as Ph-OH. Finally, my previous answer might give away, what is a Phenyl. It is a functional group derived from benzene. In other words, benzene is a simple substance, but when we substitute any of 6 hydrogens attached to it, we form a functional group, that is a part of different compound. For instance, in Phenol example, when we substitute hydrogen H to -OH group, we have Phenyl group attached to OH group, that gives us Phenol, or as I already showed, (Ph-) + (-OH) =Ph-OH, and shorthand for Phenyl is Ph- (Which stands for Phenyl group attached to something, as shortening a hexagon to 2 letters, make reactions on paper seem clearer. )