I'm 49, 200 lbs. expert vet motocrisser and been street riding since 1983 on Honda MB5s up to ZZZr1200s and three KTM 950s, and a bunch of sumoto 450-530-610 street/ race bikes..
I've had the Ninja 300, 25,000 miles, and I own an R3 and now RC. The RC turns better, changes lines better and feels about 80 lbs. Lighter than the R3.
The R3 rear shock could use more rebound damping, but the RC rear shock sucks. Not enough time turning the RC to comment on the forks, the R3 at street sport riding is unremarkable, which is a compliment after the Ninja.
Other notes I'vem made, Ninja has smoothest engine, holds far more fuel. Everything else about it is worse. From build quality to bearing style. Loose balls in steering head lasted me 800 miles. Plastic body parts are brittle and poorly designed at fitting points. Brakes marginal, tires too...
R3, more refined than RC, holds more fuel, build overall good. Solid. Front forks don't flex like the N300 did, frame seems more stable too. Nearly as smooth as the 300. Better overall street bike, wind protection and range of power/flexibility. It pulls clean at WFO from around 2000 RPM. Stupid good from 30 mph in sixth up. Caged ball bearing races in steering head same size stem as a GSXR 600.
RC, comical fuel range. Start planning for fuel at 100 miles. 47-57 mpg on mostly highway for 2200 miles. Stopping every hour fifteen on a full day ride for a week blows. Vibes much more than the other two. Power is in much smaller range of RPM, balancer kicks in about 5,000 rpm. Below that, WFO brings pounding protest thru the pegs and bars. Feels much lighter, steers telepathically and corrects lines at a thought. Riding dynamics are a hoot, but you gotta adjust to the engine delivery.
RC plastics are great, just like my 950smr. Most of the build is better, with a few wonky bits.
What I did to each to live with them...
Ninja gsxr shock, fork emulators, pipe, tuner, tapered rollers in steering head, big brake upgrade, cal/master/hoses, custom seat... A few other things. Rear rack n luggage. Should have been a better bike to match the engine.
R3, seat, R6 front brake caliper and a bigger master, SS hose, rack n luggage. I'd do the rear shock when it is wasted. Out of the box better than the modded
Ninja by far.
RC, different animal. I ordered the ergo seat but did 5 miserable days on the bike and the seat wasn't the worst part. Lol.
It's the sport only bike to the other two sporty UJM bikes. I might tour it again, but it's not great at regular street riding. A loop for an afternoon will have me stopping for fuel at least once. The battery should be down in front of the engine like the Adv 950 and they might have fit another gallons of gas in it. As a track bike that's legal it's the best if the three. (Duh I know).
I test rode the R3 and the RC390 before settling on the 390...however, my purchase was made purely to race the bikes.
For me the feel and feedback in the handling was better on the RC once I had the rear shock stiffened. The bike is more of a race-bike than the R3 IMHO.
Keep your eye on the MCRCWA web site( https://mcrcwa.com.au/ ) and watch the production cup racing this season. There are Ninja 300's, R3's and RC390's competing across two classes in the same race. Season opener is March 20. Should answer some of your questions by the end of the season.
Im hoping Kawasaki fires back and this pattern continues. I'm thinking all of them have been a sales flop though. Shame cause, in a 0-80mph public road reality, they bring more to the table in feel and dynamics that the +600cc sport bikes.
oh, and another thing that bugged me on the RC, the fit between rider and bike at the seat/tank and the point just behind and inside my knees. Very uncomfortable if you push forward into the tank. The R3 and N3 contact points are natural. The RC has weird rear tank flatness with hard corners. That and the fairing edge eats into my legs at 6-01".