These are the three main disciplines of science.
PHYSICS : this is derived from the Greek word 'physis', which means 'nature'. It is basically, in simple words, the study of the abiotic, i.e. non-living components of nature and its varied processes, which is divided into basic sub-divisions and their specific study- motion(mechanics), light(optics), subatomic particles(quantum), sound(sonics),etc.
CHEMISTRY : this is derived from words originating in either ancient Egyptian or Arabic. It is the study of various elements, compounds and mixtures existing in nature, or artificially obtained. It may have been originated from the word 'alchemist'.
BIOLOGY : this is derived from the Greek word 'bios', which means 'life'. It is the study of nature's biotic, i.e. living components of nature and its various ecosystems. It can be divided into three main subtopics: botany(plants), zoology(animals) and physiology(function of living systems).