A SQL Question About distinct, limit, group by, having, aggregate - Mailing list pgsql-sql

i have a little question, how to get 3 higher score student in every class.
Data looks like as below

problem:
id        class        score
john        a        100
jenny        a        70
ken        a        59
mary        b        85
jacky        b        80
lily        b        70
kevin        b        50
david        b        30
tina        c        85
tony        c        80
bare        c        70
vivian        c        60
eric        c        57
andy        c        50

result:
id        class        score
john        a        100
jenny        a        70
ken        a        59
mary        b        85
jacky        b        80
lily        b        70
tina        c        85
tony        c        80
bare        c        70

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