If your C-column is sorted, you can check whether the current row is equal to your last row. If not, use the current row number as the ranking-position, otherwise use the value from above (value for b3):
=IF(C3=C2, B2, ROW()-1)
You can use the LARGE function to get the n-th highest value in case your C-column is not sorted:
=LARGE(C2:C7,3)