When the R606's came out I tried the Pi 3b and had terrible performance with it. I moved up to the Pi 4 and that works great.
Just my $0.02 worth.
Was it the 3B or the 3B+? Not sure if there's much of a difference spec wise but I am running 2 R606's with 5 Compac F's & 2 newpacs on my 3B+ at my family's house with no issues. every once in a while (like once or twice a week) one of the R606's stops hashing and you just need to plug/unplug the USB from the Pi but otherwise it runs smooth. Pushing like 3.5 TH/s there on a headless Pi.
Roughly have the R606s @ 800-1000 GH/s
Compac F's @ ~300GH/s
Newpacs @ ~100 GH/s
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Can't remember which mine was for sure, but I think it was a 3B. It was very unreliable. The Pi 4 was always stable and never had to reboot, unless the power bounced. Glad you have good luck with yours.