My Airport Extreme chooses channels automatically at boot time, then never again. This is fine for my house though, because my neighbors are reasonably far away.
At work, our Meraki AP has this kind of 3rd radio, and we get very good performance in a terrible RF environment, where lots of other equipment (from Apple and Ubiquiti) has failed.
Google is doing nothing special here.
Google's spec sheet [1] specifically has 3 radios:
http://i.imgur.com/x7CMktb.png
The first two radios do MIMO 3x3, and the third radio just has 1 antenna (since it isn't serving client traffic).
While this isn't completely new, it's definitely not common, especially for a consumer-grade AP/router.
For me, at least, a 3rd radio will be on the list for my next AP, so I'll take a close look at this one, too.
The issue is just that AVM produces different versions of the Fritz!Box for every ISP, so some have more storage, or more radios than others.
So, it’s pretty much in every house in Germany.
If OnHub has to compete against this (for the consumer free) router, then it’ll have a hard time.
Because $40 routers can do that.
No $40 routers that I know of do anything like that.
So it's conceivable that it would better your Internet.
However, seeing better performance 5 GHz 802.11 n/ac requires you to have both a good router and a good laptop -- my experience is that unless your users/family blindly uses Macbooks, they won't see any improvement from a $200 router because their 2-3 year old $500 Windows laptop only has a 802.11 b/g/n 2.4 GHz-only 1x1.
If you don't have an expensive product, do some tests with and without the router's QoS. You'd be better of with a raspberry Pi doing QoS in most cases.