What are the three main rules in the Harry Potter series?

Answer 1

Thats debatable and rules can refer to anything however:

1. Do not use magic in front of muggles

Naturally, muggles do not understand the concept of magic and it would cause disruption in the world if the Wizarding World was discovered by Muggles. So, the use of magic in front of non-magic folk is forbidden

2. Do not use magic outside of school until you come of age

At age 17 students come of age - much like we become adults at 18/21 (depends on what country). Coming of age, for us, means we can drive, we can drink, we are allowed into movies and adult areas (clubs etc.) Coming of age means a very similar thing. Magic use outside of school is permitted. This is for very similar reasons - becoming an adult means that you have responsibilities and a new maturity which you can be trusted to use wisely. To use magic outside of school before turning 17 is illegal, just as driving without a license/P Plates/L Plates is illegal before becoming an adult is illegal

There are several rules in Harry Potter, just as there are several rules in our everyday life. These two above are national laws - that is, laws of the wizarding world. They are of utmost importance. There are hundreds more where they came from, just like in our world. To name three is virtually impossible