First step, get the training you can find.
A side by side RV-7 transition training course will give you 99% of what you need to fly your RV-3 safely.
Have a good Phase I plan for flying your RV-3 since it is truly a newly minted EAB. You have the challenge of needing to watch your engine and systems closely while learning a new aircraft.
Another good option, honestly, is to hire an experienced test pilot to fly the first few hours of your RV-3 phase I test flights. This will reduce the risk of your first flights when you are new to the RV series aircraft.
Are you tailwheel current? Tailwheel experienced? Go get a few hours with a CFI in any tailwheel aircraft. Recent experience and feedback from your CFI will help you make good decisions based on reality.
Sometimes a weekend trip to a place where you can get training, like down to Texas for RV-8 flying with a guy that has the correct set up for that purpose, is the most effective. Consider thinking outside the box. An RV-8 front seat will be completely different in some ways than the RV-3, but will also give you the sensation of single seat, centerline, right-hand stick and left-hand throttle feel.