Third tier PPC networks are alternative PPC networks which offer relatively low volumes of search queries on their network and therefore offer bid platforms with limited features.
Traffic quality and click fraud are often issues on 3nd PPC networks.
If second tier networks often claim themselves as being 2nd tier behind the two geants (Google and Yahoo/Bing), the 3nd tier appelation is usually not sought after.
Of course there are no clear lines between 2nd and 3nd tier PPC networks and the concept is subjective.
See also second tier network.