Issue Related to Groupwise Calculation using Formula field in Cystal Report

Question

I have Crystal Report that displays Students Markssheet SubjectWise(group by Subject Name) as Following:

ExamSubject - Maths

StudentName      Marks     PassNumber

-------------------------------------------------

Ghanshyam         89              1

Vijay                   87              3

Mayur                 88              2

Nishant               87              3

Vishal                 76               4

Jitendra               67              5

Slesha                 56              6

Hardik                49               7

I just Want to Display Top 3 Ranker's Name group footer.

How can i do this?

Note:-  There are 5 Subject in the Report, and for each subject the Renker will be different. 

 Thanks...