Hi guys,
the third renewal can be conducted at any of the medical centers listed in this document -
http://www.dgca.gov.in/medical/c1exam.pdf
The rule indeed was as mentioned above, 4th medical earlier but now, its been amended since the 3 facilities donot have the capacity to handle so many candidates on a regular basis.
The 4th renewal or the 5th medical that has to be conducted in Bangalore/Delhi/Jorhat.
cheers,
P