What are the differences between geology, petrology, and lithology?
The basic differences are:

Geology


study of rocks and minerals: the study of the structure of the Earth or another planet, especially its rocks, soil, and minerals, and its history and origins
structure of area: the rocks, minerals, and physical structure of a specific area


Petrology


study of rocks: the study of sedimentary, igneous, and metamorphic rocks with respect to their occurrence, structure, origin, history, and mineral content 


Lithology


description of the physical characteristics of a rock unit or rock formation, and actually not a scientific discipline as the two above