Science is a vast subject and can be broken down into two key areas, natural sciences and social sciences. By the phrasing of your question, I assume you are referring to the three main roots of natural science - these are biology, physics and chemistry. Each area can be defined as follows;
Biology is the study of living organisms and concerns the study of all life forms on the planet. The area can be further subdivided into numerous fields such as zoology, botany, microbiology and genetics
Physics is the study of the laws of the physical world, primarily matter and its motion through space and time. Physics also considers principles of energy and force, and is essentially the study of the building blocks of our universe
Chemistry has some interrelation with physics, in that it too is concerned with the behavior of matter and the changes it undergoes. Chemistry approaches matter in a more specialized manner however, examining its composition, properties and behavior rather than its basic characteristics.
- Physical science is the study of matter and energy, thus is mostly physics and chemistry based.
- Natural science is the study of nature and living organisms interaction with the earth, i.e. Biology
- Earth/space science is the study of the solar system, geological formations and suchlike and is mainly covered by physics although contains aspects of all three disciplines.