It's a little early. If I remember, the G2's vehicle mount didn't come out for a few months after the G2's release. I haven't seen a G3 car mount yet. No idea if they'll even offer one up. If not, you'll have to get a standard one that it'll work with.
This is what i use. I had an outlet mount in expedition but since i drive wifes new si i had to get something the fit the small space. Currently use with gs3, but it holds wife's g2 fine. It's simple and cheap, takes a while to ship though.
The overview of the one I have. Mainly needed for Navigation. I hope Honda decides to update the system with Android Auto. The demo car at the Google I/O was a 14 Civic so its possible.
UPDATE: Here is the mount I reviewed with the G3 in it. Its pretty much the widest phone that fits but it works. I have the Spigen Air Cushion case. Its pretty slim but adds a little to the width. The mount may stretch a little further to accommodate an otterbox, but it would be really a tight fit.