The Tests - Simultaneous
The next plot shows the three AC clients run simultaneously. The router didn't cough up the 530 Mbps required to support all three at their maximum bandwidths. Instead, throughput totaled only 246 Mbps with plenty of variation for all three streams as they competed for bandwidth. This is slightly below the 250 Mbps average produced by the AC1300 adapter running by itself.
Simultaneous tests - downlink
Things were a bit more well-behaved running uplink as the plot below shows. But again, total throughput does not exceed the maximum produced by the single highest-class (AC1300) client running alone.
Simultaneous tests - uplink
I also tried a downlink run, staggering the starts of each device by 40 seconds. The result below doesn't show the extreme battling for bandwidth produced by the simultaneous starts. But you definitely see an increase in the AC1300 client's throughput variation as the other clients come online.
Staggered tests - downlink
The Tests - Bandwidth Caps
With mixed-standard networks, we found bandwidth was shared more equally when bandwidth caps were applied to each client. So we tried this again, with similar results. The plot below shows each client's transmit rate limited to 50 Mbps. Couldn't get more equal sharing than that, eh?