Does Rachio G3 work with Alexa?

Yes it does. You’ll have to use the Alexa app to configure the Rachio skill.

Now, just because one can, doesn’t mean one should. I don’t see a lot of utility in having this voice enabled. I’m usually wandering my yard on those rare occasions when I want to run a zone manually. That’s a use case for the app, not Alexa. YMMV.