I am considering the same thing but making a light RV-8 with only a single seat and smaller fuel tanks.
People are confusing Light Sport Aircraft and Experimental Amatuer built aircraft that can be flown by a sport pilot.
To be flown by a sport pilot, the amateur built experimental aircraft need only meet the basic requirements of the light sport category.
If you build the engine yourself, or use an engine like the one in the Carbon Cub SS (180 hp for takeoff and 80 hp continuous as placarded by the manufacturer, you could theoretically limit the aircraft with maximum continuous power to 120 KIAS.
I wonder what the top speed of your planned RV-3 would be with the same engine used by the Carbon Cub SS (a factory build S-LSA)? That 180 hp would give you spectacular takeoff and climb performance but you would have to slow way down in cruise with the 80 HP max continuous power.
You could also go without wheel pants and perhaps add a cargo pod or something else to slow the plane down further if necessary.
I have flown with friends in their various RV's and many of them cruise at 140 kts, so slowing it to 120 kts should not be too hard. In fact the RV formation flights are flown between 120 and 140 kts.
You could not put a stock factory built engine in the aircraft and derate it yourself, from what I have read, but you could build your own experimental engine (if you rebuild an 0-320 yourself and you are not an A&P or you put non PMA'd parts in it, it becomes experimental, you can limit the maximum continuous power.
What needs to be avoided, however is "cheating" the system, where you build an aircraft that secretly cruises at a 140 KIAS versus the allowed 120 KIAS or that routinely operates over 1320 lbs gross weight. Cheating the system will only cause the FAA to crack down on all of us.
I would like to get a definitive answer on this from the FAA or EAA, but so far all I have received is off the cuff speculation and people confusing LSA rules with the rules defining eligible Experimental Amateur built aircraft.
Other posters out there, please do not speculate or through out ideas, we need facts not opinions in making these important building decisions.