To save myself a lot of typing, this briefly illustrates the 3rd spring.
http://www.optimumg.com/documents/techt ... _Tip_6.pdf
Just like the ARB is there to provide additional roll stiffness while not changing ride stiffness (though it is coupled with single wheel bump).. a 3rd spring, aka ride spring, aka "pitch spring" (but not really) is there to be activated in ride and pitch but not in roll.
Helpful for high downforce cars, you can use 3rd spring to prevent the suspension from totally compressing on itself. Or if in worst case scenario you can use 3rd spring packers to absolutely limit the amount of compression.
If your diffuser works really well at high camber angle / lots of chassis pitch you can do some trick stuff with 3rd spring to that end.
Regarding G-G plots ("friction" plots).. its a metric for determining how well your driver is pushing the car. How close to the limit. In a perfect world, you operate at the limit of the tires at all times. They can produce all longitudinal force, all lateral force, or some combination of the two. The vector sum of those orthogonal accelerations is a measure of the total acceleration of the vehicle. Lets you show how well you are trailbraking and accelerating out of a turn.
A perfect corner would be a half circle. Starting at (0,0) you would slam hard on the brakes to (0,MaxBrakingAcceleration). As you turn in you gradually add steering while letting off the brakes. You go around the outside arc of the friction circle to (MaxLatAcceleration,0), while on the apex. At the apex all your car should be doing is max, all out pure cornering. From there you unwind the steering while getting on the gas to sweep around to (0,MaxDriveAcceleration). At all points you are using the maximum accelerative potential of the tires.
If your G-G plot is more of a cross ("+") rather than a circle ("o") it indicates you only use the brakes in a straight line, and can be trailbraking harder and getting on the gas earlier.
Realistically its not likely that you're going to really use the whole friction circle/ellipse/blob. But you can get somewhere between a diamond and a circle and that's pretty quick.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.