So FYI I had this issue for several hours too and I will share how I solved it for myself in hopes it might help someone else, though I don’t know how. I have a rather uncommon setup - Windows default firewall set with a DEFAULT DENY OUTBOUND rule. Only programs I whitelist can access the network/internet. I did however whitelist inSync and never had issues with this in the past.
When trying to log into inSync 3 for the first time I could not login, the oauth page just sat at the “connecting to insync” page like OP. However I disabled the firewall momentarily and boom, login successful, app working (kind of). The app still won’t sync when I have the firewall on, but I suspect this is a deeper issue as I see successful connections to some Google and Digital Ocean IPs from inSync.
But in case anyone else has an outbound firewall, inSync is doing something funky with networking (maybe using a service of some sort or Microsoft BITS or something?) which may require disabling the firewall.