I'm going to preface this with a caveat: I'm a little bit pissed off. I bought a Bip 3 watch because I wanted to link it to Gadgetbridge and keep health data local.
Unfortunately it's turned out (jira-projects/MISC#34) that that's not possible: the manufacturer appears to have changed the underlying firmware and Gadgetbridge doesn't have support for it. That'd be ok(ish) if the Zepp app provided a way to access the data without having to give it to Google Fit.
I've raised this ticket to track efforts in getting something up and running