Hello,
the relationshipt between densitiy and flow comes from the interaction between vehicles. There are basically two components to that:
1) interactions in car following due to differing desired speeds (which includes slow-downs and lane-changing). Unfortunately for your case, sumo vehicles have the same desired speed by default so you will net see any effect unless you change vType parameters. I recommend setting speedDev="0.1"
2) Intersections. (i.e. slowdowns when flows merge on a motorway, lane changing to take the exit, waiting at a traffic light or some other intersections where right-of-way must be considered.
If you just insert default vehicles on a 3-lane street you will see neither of these effects.
On a side node, SUMO alone is probably not the right tool to figure out the maximum capacity of your roads. The value you will see depends on the calibration parameters of the models and the default parameters may not fit to the driver behavior in your city. I suggest you complement your research with some real-world data.
regards,
Jakob
