What are the three branches of science?
Legislative, judicial, and executive...? No, I got mixed up...
In the context, you probably refer to:
I would say physics, chemistry, and biology.
In terms of content, natural sciences might be able to be split into three different major branches, such as physics (the study of energy and forces), biology (the study of life), and chemistry (the study of matter and reactions).
Other branches such as geology, mathematics, and astronomy can are mixed up subjects, such as physics and chemistry fused together. Mathematics however, is an exception, and relies mainly on logic, and does not rely on the "big three".
Now for the social sciences, like political science, history, economics, and geography, I do not consider them real sciences, as they mainly do not follow the scientific method.
Hope this helps!