3Rensho's were built for several years by Yoshi Konno on his own. He got busy and added a few builders, who later came to the US or sold here a lot and promoted the Yoshi Konno roots. Back in the late 70's, I think it was, 3Rensho's were coming into the US and were nice but not considered spectacular frames -- that was reserved for Masi's, Confente's, a few domestic builders, and even bigger builders like Colnago. Nagasawas were occasionally around, but mostly as a legend ridden by Nakano.
After his accident, a few of his builders kept the shop going for a couple years before starting their own brands. The place was bustling at that point and they had some assistants to help with brazing and finishing (more than Konno had, and not the quality builders that Konno recruited).
I was lucky enough to manage a Nagasawa and an old Kalavinka, and have a couple older frames as well (Sachs, Confente, etc.). The Japanese keirin frames are all very 1970's style European track design -- relatively steep, quite tight, and fairly short trail. They are a compromise between the huge long tracks that keirin is raced on in Japan and the rather twitchy track frame designs from Europe that Japan was so enamoured of at the time (and that then became memorialized in the classic keirin design). They become increasingly stable at speed, but they can sometimes be a little erratic at less than race speeds (a little too twitchy on the road, and on the track they don't keep a line as well until they're up to speed).
Nagasawas are pieces of art in addition to exquisite racing tools. Worth it at $2385, which I believe is still the current price for frame/fork/headset/bottom bracket. Still worth it at up to about $3000 (that's around what you pay for a Pegoretti BLE or a Sachs, after all), Kalavinkas aren't quite as nicely finished, but pretty darned close and both better priced and available without a long wait. Tanabe has also moderated the designs a bit to reflect your actual riding or racing conditions, so they race and ride as fixies really well. I've ridden an assortment of other Japanese frames and can't say there's a bad one, but the finish quality varies a bit and of course the tubings under the paint vary as well. We get very picky about the tubing in a European frame, but no one ever seems to ask about a Japanese one. The difference between a 8630 and an 019 frame is huge -- in stiffness, responsiveness, and weight. Some of these frames are like mid-range Trek's -- they really should only sell for $350-600. It's a place to be discerning about the frame -- the NJS certification doesn't mean they're all built to the same quality level.