Monochlorophenols have three isomers because there is only one chlorine atom that can occupy one of three ring positions on the phenol molecule; 2-chlorophenol, for example, is the isomer that has a chlorine atom in the ortho position.
The presence of multiple forms of monochlorophenols stems from the fact that only one chlorine atom can occupy one of three distinct locations within the phenol structure; 2-chlorophenol serves as a representative example where the chlorine atom is situated in the ortho position.