You've asked to substitue the nth occurance, and you've asked to substitute the occurance on the nth line. You've been provided with a solution the latter; what follows is solution to the former.

perl -pi -e "s/searchpattern/replacepattern/
   if (/searchpattern/ && ++$match==3)" filename
[download]

hum, let's eliminate the redundancy:

perl -pi -e "substr($_,$-[0],$+[0]-$-[0], 'replacepattern')
   if (/searchpattern/ && ++$match==3)" filename
[download]