The Tests - Bandwidth Caps - more
With around 250 Mbps of bandwidth available, I should be able to set the caps at 80 Mbps and still have all three clients sharing nicely. So I naturally set the limit at 90 Mbps and got the results below. While none of the clients hit its cap, bandwidth was divided surprisingly equitably.
Simultaneous tests - downlink- 90 Mbps rate cap
To show this nice behavior has its limits, I raised the bandwidth caps to 100 Mbps and got the result below.
Simultaneous tests - downlink- 100 Mbps rate cap
Once again, in no case did total bandwidth exceed the 250 Mbps best case obtained with the AC1300 Mbps client running solo.
We shouldn't be surprised by these results. Channel capacity (bandwidth) is determined by the Modulation and Coding rate (MCS index) and number of spatial streams in use (see chart). In the case of our test router, the maximum usable throughput was measured at ~ 250 Mbps with a client capable of linking at the router's maximum 1300 Mbps 5 GHz rate. So even if we had all AC1300 (3x3) AC clients, they still would have only 250 Mbps of bandwidth to share.
So the answer to the question of how much throughput you can get from an AC router is: no more than the maximum supported by the combination of your router's class and its highest-class client. Adding more clients, even if they support the router's maximum rate, only divides the available throughput among them.