Why science has divided into three branches?

Answer 1

Quite simply, it isn't.

In the 18th and 19th century, most scientists were polymaths - the same scientist might examine questions in Biology and physics and then go on to look at geology or chemistry without thinking that he (and it usually was a he) was looking at a different subject.

As we came to know more of how the world worked, it became convenient for people to specialise in certain areas of knowledge - to be good at chemistry involved so much chemical knowledge that it was no longer possible for a single person to know all of chemistry AND also be much good at, say, Geology.

At that stage, scientific study was split into areas relating to different areas - physics, chemistry, biology, geology, zoology, medicine etc., mainly because the areas were easier to teach that way.

We are now at the stage where no normal scientist can understand all of any single subject, so a chemist might specialise in physical chemistry or inorganic chemistry, and even within that specialisation, they will become an expert in only a small area.

The best science now involves several scientists collaborating - bringing their expertise in chemistry with someone else's expertise in biology and another person's expertise in medicine, for example, to aim at a cure for a particular disease.

Finally, don't think about science being a subject - it is a method of thinking that allows you to put up an idea, test it with an experiment and see if your idea holds water, whether it needs to be improved, or whether it is wrong and needs to be discarded.