Axel, there is a relatively short list of good alternatives, among them, Wilburs and Hagon Nitros, in addition to the most common/popular shock upgrade which is to Progressive's 444s. I am using Progressive 440s, the precursors to the 444s and they work just fine. Most R3 owners, other that the very most discerning riders, do not upgrade the front springs - finding it to be unnecessary for our everyday riding styles/conditions.
I use the Bridgestone Exedra Max rear tire, which as a 240/55 16 has a 0.47" taller profile than the stock Metzeler rear. Since it is paired with Progressive shocks left over from one of my Honda VTX 1800s that are 1/2" shorter than the OEMs (12"), the combination leaves the rear geometry essentially unchanged as from the factory, but I do run a matching 130 tire up front and that significantly improves tracking, turn-in, overall handling and maneuverability, making my bike feel about 150-200 pounds lighter and resulting in an eminently
flickable bruiser cruiser.
Because a 130 tire will lower your front end and, hence your ground clearance, it has been deemed unsafe and unsuitable for the Roadster due to its factory ergos. Also, due to the tires decreased circumference, the 130 defeats the ABS brake system, further making it unsuitable for models so equipped. Nonetheless, you can gain most of the performance and handling benefits of a smaller front tire with a 140/75 17, the best of those available being the Michelin Commander II, IMO. The improvements attributable to it will be just as stark as you will experience with a good rear shock replacement! The OEM 150 up front is strictly for looks and not for performance, again, just my opinion based upon direct experience.
Point of information - there are some good threads here abour rear shock options. Use the search function or Google (Triumph R3 rear shock options, for instance) to source them!