I have database of students.. I have generated Student Exam Report with Marks and PassNumber
as displayed in Following Image

Report had grouping field Group by Date and ExamNumbers there may be 20 pages in current report, each group displays on New Page.
Now i just want to displays name who are TOP 3 rankers among Passnumber field.
how can i do it using formula Field?
You can conditionally suppress the details section for students with passnumbers > 3.
Go to the Select Expert for the Details section and add this as a conditional suppression formula:
{table.pass_number}>3. This will prevent all but the top 3 (including ties) from being displayed on the report.Edit: If you just want to display the names of the top 3 students in the GF, you can add them to an array in the details section while processing the report. To do this you’ll need to create a formula in the GH that declares/reinitializes the array, a formula in the details section that checks the pass number and puts it into the array if less than or equal to 3 (since there can be ties, this will get a little tricky and you will have to resize the array if necessary). Finally, in the GF you can display the array with a third formula that joins the array’s elements together via
join(myarray,', ');