The three plot radius is fun, but has some sideeffects worth noting. First off, the AI doesn't use the concept. Their cities are placed too close together to really benefit from the increased radius. I recommend you alter the files that effect city placement, and increase the minimum from two tiles apart for cities to three tiles.
If that can be done, then its a must! As it was with civ4 (and rom) before, the AI didnt even benifit from the second radius, but putting cities way too close with too much overlap (IMO). I, personally, dont plan cities for the 3rd range, but put them in "normal" range. But sometimes a resource pops up with a tech, and then its nice you can use it instead of just having a useless improvement. BTW I found another feature/bug : When a city only has culture level 1 it cannot use second range even though its inside cultural border.