3 teams from Marion County is overkill. Look at the census data and it has less than 1 million . Lake county, the next largest has just under 500,000. If you add all the counties together that all of the schools attend the same regional they have a much larger population than a county that is split into 3 different regionals.
http://www.stats.indiana.edu/population/popTotals/2008_cntyest.asp
The counties of Lake, Porter, Newton, Jasper, LaPorte and Starke have approx 200,000 more inhabitants and they never have the opportunity to send more than 1 participant to team state. Marion County always has 3 opportunities.