I just received my 3a and am a Google Fi user. I was unable to activate my sim though the phone. Google said they could not help me any further in the activation process since the nothing 3a is not officially supported (yet?). I may try to activate my sim card with another phone, but not sure if it would work.
UPDATE: so after some troubleshooting, I discovered the sim card was sitting too deep into the tray, failing to make contact with the pins in the phone. I was able to install the card in a way where it successfully activated the sim card, and can now use google fi on the 3a.
I’ll be testing it this weekend.