Please explain to me

What is a difference difference between among these words: horrible, terrible, and awful? Are they synonyms?  Maybe they are just synonymous. I have heard that they have different shades of meaning sense. but I don't understand in what case each of them they are used.

They are synonyms with "awful" being the general case.  It means extremely unpleasant.

Horrible: causes horror but there is no fear.

Terrible: causes terror, fear but doesn't mean "ugliness".

She died a horrible death.  - perhaps her face was so mangled that it causes horror in you.

She died a terrible accident.  - The way she died causes fear in you.

You looked awful.   --  doesn't mean that you are not beautiful but just don't look good

You looked terrible --  causes fear in others because the look in your face.

You looked horrible --  you are so ugly that it causes horror.

In general usage, especially in speech,  they are all mean the same.