when I learned about the difference between those. I remember these, if it's neccesary to do, we will say 'must do' and if it's an our opinion, we will say 'should do' or 'ought to do' but 'ought to' have more neccesary than 'should'

Ought and should are beliefs, some would say strong beliefs, that something is to be done. Must is something you are required to do. "I have not eaten all day. I really should get some food (or, I really ought to get some food.).". If you do not get food, you will still be hungry, but you will not "die" so it is not a must. "I have been locked inside of an abandoned building for 10 days without food. I really must get some food.". This situation is more of a requirement than the last one. "I want to be a doctor, but first I must graduate from medical school.". This is not a should or ought situation, you need to graduate medical school before becoming a doctor. Be aware that many people use should, ought and must interchangeably. Listeners will know if it is truly a requirement.